I think there is some truth in them going back to previous work or ideas, in part because there is great sense in doing that, logistically and financially. But just because some of the pass will be like that, it certainly doesn't mean it all has to be.Yeah it's a little shakier, I think the other three characters affirm the "leftovers" idea much better than Sephiroth (I kinda hate calling it that but you get the idea). You could argue the fact that Sephiroth's name was coming up that long ago supports it to some extent, but I don't really think it's an all or nothing sort of thing anyway. That's just why I assumed and considered Sephiroth to be at least a defensible part of that theory.
And I think people are being a little quick to lend credence in the form of extremely generous benefit of the doubt to what is, in effect, an index of claims, so as to make it fit with the others. But keep in mind that Sephiroth doesn't have to fit. Not every character has to be one with earlier roots.
And even if he is, it's still dubious to impart that list with its current credibility, considering its origins, timeframe, and purpose. That's my main issue.