They've gone back and forth being classified as mutants and mutates for their entire publication, so the short answer is that the origin of their powers is whatever makes them useable for the comic they're in. Even when they were recruited by Osborn for his X-men team they admitted that they didn't think they were, and Osborn said something along the lines of 'the public thinks you are, that's all that matters'. I just decided to go the magic/chthon angle because a lot of what's known about the Darkforce Dimension supports it, it really simplifies things, and I think it'll be interesting to explore.

I've never actually read much of the Mangaverse. Have any recommended titles?