There’s a lot to unpack about why the setting and the game package itself is colonialist and it is both because of who is writing the game and their perspective and also the perspective of the narrator(s) that the reader assumes by reading the books. Symbaroum’s writing is diegetic, so all of the text is in-game or in-world (even the mechanics explanations are flavored).
Symbaroum was a product of Järnringen, a Swedish company, which has been be bought Fria Ligen, another Swedish company. The game is written from a Western European perspective, which is historically colonialist. Fria Ligen, therefore, is part of this history of colonialism, even if Sweden as a nation has not actively colonized (but the Nordic countries are certainly no strangers to the idea).
The game corrupts high fantasy tropes to create a dark fantasy setting. This still falls well within what I call “western fantasy,” which relies on a high fantasy model injected with influences almost solely from Western Europe, primarily from Western folklore, but also the sociocultural thought of the West — consider the aesthetics of high fantasy and then why the genre tropes of high fantasy seem not to apply to other cultural contexts. It’s important to remember that folklore has an agenda: not a single folktale (including fairy tales) exists that is not also an allegory and/or metaphor.
So all of the Swedish writers, presuming they have grown up Swedish and have been enculturated to Western ideals, are writing Symbaroum from an explicitly Western perspective. This is an important metatextual aspect that we can see across Western works — consider how poorly Kara-Tur and Chult are portrayed in older editions of D&D and how Chult in 5e still isn’t a great representation. This touches on your question whether all traditional RPGs are inherently colonialist — I think no, but I’ll go into more detail later.
Now we need to take a detour to talk about the setting of Symbaroum and, specifically, the peoples who inhabit the setting.
First some notes about the setting, in response to your observation from Fria Ligen’s pitch: The setting is not (post-)apocalyptic. Symbaroum is an old empire that has not existed for centuries, whose ruins are beneath and within Davokar, a sprawling forest allegedly planted by the Elves (who have a natural mandate to protect the balance of nature) to control the corruption caused by Symbaroum. Symbaroum was a generically evil empire that used magic — which has a cost of corruption, a sort of degeneration of one’s humanity — to increase their power and control. They took it further and also abused the corruption itself for the sake of power. This is what led to their downfall. All Humans are presumed descendants of the Symbar people, whose empire stretched far through imperialism and retracted as power was lost and concentrated to the center of the empire (what is now the forest Davokar).
South of Davokar is a region of plains and then a mountain range, beyond which is a country called Alberetor. This is the ancestral home of Ambrians. The Clanfolk are from Davokar and primarily live on the outskirts just inside of the forest, but they are nomadic. Some two decades before the game, a war devastates Alberetor, literally killing the land (magic again), and forcing Alberians to leave. They go north, over the mountains, and “discover” the fertile region just south of Davokar, which is inhabited by the nomadic Clanfolk (the only non-pejorative term used to refer to the “barbarians,” a word which is a racist pejorative and it has literally always been so. It’s used to describe some Other who is placed in opposition to you and assigned traits like cultureless, aggressive, etc.).
Let me give a brief overview of the peoples before continuing:
Ambrians are a standard Western European people — they have knights and kings and nobles and castles and a monotheistic religion. The Ambrian perspective is the game’s default perspective and, except for a few rare exceptions, the only perspective. This is important to remember because, as the text is diegetic and presented as if from an in-world narrator, all of the text is written from an Ambrian perspective.
Clanfolk are presented as the Other with their tribalistic clans, polytheism and lack of permanent settlements. While Ambrians have a central identity, Clanfolk identity is clan-based and comes from many reductive cultural influences.
Elves take on another Western European folkloric trope as protectors of nature and balance.
Aside from Ambrians, Clanfolk and Elves, there is another group collectively called the Elder Folk, which the Elves are part of, along with Changelings, Goblins, Ogres, Trolls and Dwarves. These are like a Super Other, all of whom are perceived as far more foreign than Clanfolk. For this explanation, however, I’m ignoring the Changelings, Goblins, Ogres, Trolls and Dwarves.
So, as refugees, the Alberians (soon to become Ambrians), commit genocide and wipe out one of the Clanfolk clans, subjugate and assimilate another, and then claim the plains region south of Davokar as their own country. They name the country Ambria. You might draw connections to the Western European colonization of the Americas. The Ambrians, as new people to region of Davokar, barely know who the Clanfolk are (they don’t even share a language) and had probably never seen an Elf, if they’d even heard of them.
From here, a tripartite conflict emerges as tacit diplomatic maneuvering and balancing between the Ambrians, Clanfolk and Elves. The Ambrians, since losing their homeland, have converted almost entirely to a monotheistic religion that perceives the overgrowth of Davokar as a personification of evil, which must be (literally, to some) burned out. You might draw connections to the Crusades.
The Elves have an ancient imperative to keep the corruption of Symbaroum guarded and controlled, which they do through Davokar, the very same “sprawling overgrowth” the Ambrians want to destroy. The Clanfolk are closely aligned with the Elves, since they have lived close by (but never integrated) for all of time, essentially. It’s important to note that the Elves have told the Clanfolk and Ambrians why Davokar exists and shouldn’t be messed with, but the Ambrians simply don’t believe them and think they’re wrong. The Clanfolk just don’t have the memory to know it’s true (being short-lived humans and all).
Let me reiterate that all of this is presented from an Ambrian perspective. The Ambrians feel they have a god-given right to the region they claim as Ambria and that the “barbarians” are lesser peoples. They think the Elves are combative and aggressive, another type of “barbarian” who are just pushing against them because of rumor and conveniently anti-Ambrian sentiment. The history as I presented it is not written out so plainly by the Ambrian narrators.
The text is inextricable from both its Western writers’ perspectives and the forced colonialist perspectives of its Ambrian narrators. The writers’ perspectives are inherently colonialist because it defaults to presenting the world from a hegemonic, imperialistic society and culture and reinforces that by Othering the non-Ambrians of the setting and the setting itself both textually and metatextually (by what is included and the choice of how to include the information). Davokar is foreign and impossibly dangerous, despite Clanfolk, Elves, Goblins, Trolls and Ogres living in the forest. The Ambrians become a player vehicle for colonialist fantasies like touring and conquest. Even from Fria Ligen’s pitch: "Visit the barbarian clans to trade or to plunder their treasures!"
Now, that is a heavy critique of the system, but I think Symbaroum is still good precisely because it lays its colonialist, imperialistic cards on the table. The only problem it has is not making this explicit; there is no guide for how to engage with Symbaroum critically as GMs and players. That’s another argument for if a game has that responsibility — I think yes, however, because this game is infinitely richer for everyone at the table if they hold each other accountable to this default perspective. If you want to see what I mean (and if you have time), I recommend listening to the seven session series I did for the creators’ introductory adventure “The Copper Crown:” https://www.youtube.com/playlist?list=PL8DZ9ihhxSJzMG-mSnVTGNsEjeZ5vL7Lk. There are ways to engage everyone at the table critically without overloading players, but still asking them to engage with what it means to come from an imperialistic culture — or, if a player chooses not to be Ambrian, what it means to be(come) subaltern.