In a way, yes. Since his Nobody is in the Organization, that is true...
***SPOILERS***
Xemnas sort of marked Ansem as Number 0, as a small joke. For that reason, and the fact that he thought it was Ansem's fault in the first place, he only felt it was right to rank him. But even though he is 'in' it, it doesn't mean he is ACTUALLY in it.
***SPOILERS***