You forgot that part :
[Ansem's dark monster disintegrates around him. It reaches out for Riku, and Riku holds his Keyblade in front of him. The monster dissolves into thin air and sparks of light. Riku sighs.]
Riku: The power to protect what is important.
[He lowers his Keyblade and smiles.]
Anyway I agree since I pointed it out the moment Krexia translated it !
Also, since Terra's conditions have been the result of Xehanort's doings, the fact that Terra himself was "left" behind in several places (Ls, Ansem, Xemnas, Riku) actually makes sense since Xehanort did too in a way.
But Xehanort considers him on his side. It is possible he refers to the normal Terra, but that would have became part Xehanort. Not having Mx inside of him anymore, more like Isa.
Though he'll obviously break free, either thanks to Aqua (the one who allowed him to use his light to outmatch Xehanort) or Riku (the one who seems to share a strong bond with him). Probably one of those two with Sora's help. Maybe the 3 of them.
Or maybe Mx considers Xemnas to be "Terra on his side". Hard to tell. I don't think Terra himself is within Riku (since it was stated those in torments were rather within Sora's, thanks to his special heart), and we've seen Terra was indeed connected to Sora. But the key to his saving will probably be Riku (and Aqua depending on when that happens).
Cute scene incoming, with Riku saving the one who believed in him OwO .