I think Bonemass is a part of the swamp because of the sheer number of undead there. Not entirely sure what the connection actually is, like whether Bonemass was another being cast into Valheim and made a forsaken that just gravitated towards the largest abundance of undead or maybe Bonemass is why those draugr forgot to die in the first place or what. But Bonemass themselves seems to be an amalgamation of corpses, not any singular being risen from the dead. Even the runestones warning the player describe bonemass with things like "Wanderer, look to your feet, that tread upon our tomb. One thousand bones without their meat, will drag you to your doom," which would make it seem like bonemass is an entity that just sort of acquires/absorbs what is around it to add to itself. Especially if you want to be picky about the word choice and notice specifically the "that tread upon OUR tomb" part, implying the draugr are incorporated into this mass.

Though if bonemass essentially just feeds on the dead/undead it is also totally possible that includes more than just the draugr to. The draugr are just one race that got horribly destroyed in Valheim. I imagine if a few giants also fell anywhere nearby, bonemass would have interest in those corpses to, maybe even a special interest in them considering they seem to have special properties. If bonemass has a special interest in Jotun corpses, then it could totally make sense to make the summoning alter out of one's skull, and offer large ancient bones (that may also be Jotun potentially, certainly larger than a draugr femur) to actually peak their interest.