Yeah, that's my headcanon as well until proven otherwise. The fact that they are the reincarnation of two beings that were created purely from a demonic essence would absolutely explain why they have powers. The X-Gene explanation is ok, but I never got why some people associate that with Wanda, considering they are not her biological children. If they naturally inherited the X-Gene, it was from the Kaplans/Shepherds, not Wanda.