Oh, it's better than that. They control an area the size of the Realm without access to the ground. Anyone who knows anything whatsoever about the dependence of a large society on agriculture will instantly, instantly call bullshit on that one.
And then yes, they're a super special snowflake barbarian culture who live in trees but have advanced bureaucracy and high rates of literacy and good health and they're liberal and accepting of beastmen and they have gods and First Age Lunars on their side and the Linowa are fighting them with Realm aid so they're (one of) the LONE UNDERDOG(s) STANDING UP AGAINST THE REALM, and...
... it's nauseatingly Mary Sueish, frankly. Not to mention completely fucking stupid. At best, you should have disparate groups of "Haltan tribes" in the same way that you have "Amazonian tribes" - no unification, just a common environment, with thaumaturgic wards on their villages (often but not always arboreal, because that makes them easier to defend even when the wards don't work) that make them fairly safe from the raksha bands native to the area. They're not at war with Linowa as a whole, because there is no whole, but the tribes near the Linowan border attack them and are attacked in turn, and while they're less advanced they have guerilla supremacy in the forest - rather like the US in Vietnam. There should certainly be none of the "enlightened and accepting", "talking animals", "high literacy", "advanced bureaucracy" crap going on (though beastmen are, frankly, their own problem), because that's just Mary Sue on top of Mary Sue.
And hey, with the monopolity that is Halta broken, there's space to look at real-life historical "high latitude forested cultures" - parts of Russia, Sweden-Norway-Finland, maybe bits of Canada, the northern-most bits of Japan, etc - and put a few kingdoms and realistic societies down in that region. Ones that, you know, don't suck.