No, it wasn't Terra. It was just some part of him that was left there.
Terra was within his body.
That doesn't mean he's not "a" Xemnas.
When Terranort stabbed himself, Nomura said "it's not clear yet" if Mx and Terra's nobodies have been created or not. The "yet" is pretty much saying one day we will know (and if there was nothing at all, it would be pointless)
So maybe some sort of new being have been created at that moment...
And let's be fair, if Xehanort is coming back for his "final battle", Terra has to come back too. Especially since Nomura showed us "young MX", he'll probably get his youth again, but Terra probably will get his body back before the final battle, so Sora can fight the real Xehanort, and not Terranort.