No, Marco speculates that it could've been from his abnormal body. He's assuming that Blackbeard ate the second fruit for that to happen (since eating = places a Devil inside the body based on One Piece's theory). So we don't really know what happened.
Having said that, I don't think his abnormal body plays that much of a role, and I think it's more likely that Blackbeard extracted the powers with the help of the Yami-Yami no mi.
I Disagree. The general consensus we have on the AP boards is that blackbeard used the yami powers to somehow get the gura powers, yes. But to have 2 abilities at the same time is due to his own body or lineage of being a D. Oda wouldn't write a comment like that for Marco if it wasn't somewhat true.