That's how the game explains it. Ellone tries to save Laguna and uses Squall team to alter history to help him but all she ends up doing by meddling is making sure history stays on course cause it counted on her try and meddle with it. History is basically unchangeable.

Void was suggesting that their is an alternate timeline where Squall failed at a certain event in VIII's story to help Rinoa and in this timeline Rinoa eventually became Ultemacia; but if history is unchangeable and any attempt to altering it makes it happen then this scenario is impossible cause VIII's Law of Time Travel would suggest Squall would either always fail or save her depending on which event is true. VIII's story never mentions alternate time lines being possible. VIII's time travel is pretty straight forward and is very similar to Greek tragedies, in where the hero learns of a terrible prophecy concerning themselves and in the course of trying to change it, they make it come true.

I felt VIII was rather thorough on explaining how time travel worked in its world. I don't understand why people insist on placing alternate theories on how time travel works from other fiction into the VIII mythos.