I THINK, and also hope, that the Sephiroth defined as unknown in the Ultimania*, was making his one last ditch effort to alter the timeline. Namely that he's from post AC, and the only option left available to him was to fuck up fate and just hope that things go different for him this time, but is otherwise unable to interfere again.
*The Sephy who is ostensibly from the future, who is the one who shows up in Chapters 2 and 18. While the ones who show up as a pure hallucination to Cloud or as possessing Copies are the in-timeline Sephy.