Technically DS9 never say they exist at all points in time. DS9 says that they don't experience linear time. One way I thought you could interpret that is that

Technically DS9 never say they exist at all points in time. DS9 says that they don't experience linear time. One way I thought you could interpret that is that the wormhole aliens in fact only exist when the wormhole is open - their existence is only in those blinks of reality that they perceive, and as a result to handle the universe they have a vast predictive ability that looks like future-knowledge.