Actually Xehanorts heartless is the Ansem in KH1
and Xehanorts nobody is Xemnas in KH2
That's not what Gabranth is saying. He is stating that Riku never became a Heartless, and thus could not have a Nobody. You can't just become a Heartless of gain a Nobody if you semi-immerse yourself in Darkness. That completely goes against everything KH has been trying to explain within the second installment.
That goes for your 'theory' too. You can't gain either if you haven't lost your Heart. Ansem the Wise, so we know, never lost his. If he did, then the Ansem we see in 2 would have been a Nobody, due to the fact he has memory of everything. And to further disprove your theory, he can't even be a Nobody (that being the only alternative), because he mentioned himself that he was consumed with rage, which is blatantly an emotion; something which Nobodies cannot experience.