Zack is puzzled for a moment - does he think it was a fakeout or something? Maybe that's the logical conclusion. True revivals aren't really a thing, after all. Disappearing and being "presumed" dead is common for Shinra, too. Memos had gone out for Angeal, Genesis, and even himself that they were KIA well before any of them came close to dying. (Sephiroth too, apparently, but he was gone for five years the way Zack was, so that's fair enough.) He did die, though- it's just that the fox can bring back the dead in this world.

He thinks about it, but before he can come up with a response, Sephiroth continues, and that makes Zack smile. If he's allowed, huh? "Is that your way of saying you want to be friends? Because I'm all in."

Even if... what happens, happens... that's later. Sephiroth was a dear friend to him before things went south. He wouldn't mind extending a hand to his younger self.