I'm pretty sure it was also mentioned in Book 4 that some Oular started living on the ground, singing to the soil. Making the Oular, Grubbers, and Banda all related.
A species came from elsewhere, ended up living closer to other species, and that makes them related? That seems to indicate they are not related, at least not in taxonomic way. Or, are you saying the fact that they're also using a kind of earth magic makes them related? That too doesn't make them related. What do you mean exactly?