Um, it wasn't a plot twist. You find that out well in advance and it was forshadowed before that. Besides that, Tidus doesn't really not exist. As a dream of the faythe, he exists within the conciousness of the faythe. The faythe are now resting from their dreaming but that does not mean that there is no possibility of a return of Tidus.

(FFX-2 spoiler)

Judging from the end of chapter 2 of FFX, not all the faythe have stopped dreaming just yet.