lol sure - I was being a tad facetious in that thread given the sheer ridiculousness of the people taking the whole theory thing waaaay too seriously.
My theory is based on having absolutely NO knowledge of DMC coz I don't play it and I have no love for Wikipedia, the home of incorrect knowledge
Basically, I don't bother with the whole "is Nero related to XYZ?" thing coz it's too complex for me - I've always maintained Nero as being the subject of demonic experimentation by the Order and his arm comes from components of Nelo Angelo's whatever-they-used, hence the ability to wield Yamato etc. The whole blood of Sparda stuff/family ties comes from that arm and the essence of Vergil encapsulated within it. It's a game, after all, and doesn't have to follow regular science/genetics so it's entirely possible. And most likely horribly flawed but I don't care
My real bet is that even Capcom aren't sure how to explain Nero just yet and are probably laughing themselves stupid over the guessing and theories of fans the world over...