He doesn't doubt Sephiroth means it. That's part of what scares him, keeps him from laying it all out right in the open - because he doesn't know how to do any of that without the risk of history repeating itself. Or starting to, sending Sephiroth down the same eventual path to madness and maybe-death.

But it's also the only chance he sees for something to change. Somewhere. Even if there's no hoping of saving himself, or of saving the Angeal, the Genesis, the Cloud, Tifa - everyone that he knows - if there's a chance to save any version of them that might exist, that could, somewhere out there— He wants just as badly to help that happen.

"I know," is all he offers in return, for a moment, meeting Sephiroth's unwavering gaze with an even steadiness. Then, he relents, "Which is why you need to get to Banora - before Genesis, before Shinra, anybody. There's a lot of bad stuff going on, and Professor Hollander is behind too much of it. If you can find a way to stop him, before he makes things worse, then maybe..."