Well, we can't say he isn't. But, there is a chance he might be a part of the equation until we get to see what he looks like and what happens to him in the end. Terra and MX are also part of the equation, but who knows if there weren't three people involved.
Whoever said Terra isn't Xehanort is entitled to that belief, but with the clues presented, he has to be at least a part of Xehanort.
Whoever said Terra isn't Xehanort is entitled to that belief, but with the clues presented, he has to be at least a part of Xehanort.