Everywhere is tribal homeland. This means tribals won't assimilate but also feel "at home" everywhere.
Assimilation happens to an accepted pop outside it's homeland. This means tribals can assimilate into non-tribals if they are both accepted and not in a homeland.
Thus, there could be laws (and decrees? Random events? A completely different scripted option, like "state tameness"?) that remove the homeland status for states.
Everywhere is tribal homeland. This means tribals won't assimilate but also feel "at home" everywhere. Assimilation happens to an accepted pop outside it's homeland. This means tribals can assimilate into non-tribals if they are both accepted and not in a homeland. Thus, there could be laws (and decrees? Random events? A completely different scripted option, like "state tameness"?) that remove the homeland status for states.
See:
remove_homelandeffect.