You can't prove that the ghost is a 100% glitch though, I can't either. But the Zack's Ghost being appeared in AC makes it more plausible for Aeris's Ghost to appear in the game that it's not a glitch to some extent because all the stuff are there, go figure. Also, like I said, the game is almost flawness (almost no glitches) so it's kind of fishy to point at Aeris's Ghost and declared it as a glitch.