there's something to be examined with how the darkners in chapter 4 perceive the passage of time differently, with each fountain's denizens understanding their counterparts as being in the past
Maybe that's how Ralsei and the prophecy works too. His fountain was opened first, but from his perspective, his fountain was the last and all these events have already happened. Each time a new fountain gets opened its like he's being pulled to a different part of the past and the future has refused to change