It makes perfect sense, and is the only logical conclusion.

We agree that Ultimecia cannot change her own past, right? Well, Ultimecia's past consists of Squall's future from his own perspective. Hence if Ultimecia can't change the past, this means that Squall's future is set in stone. Hence he couldn't have changed his future because that would mean changing her past, which is impossible.

The conjunction of not being able to change the past and the fact that the future exists in the world of FF8 (evidenced by timetravel) implies directly that all of time exists "at once" so to speak, which directly implies that there is no meaningful distinction between past, present and future. If you can't change one of those, you can't change the others, because your past is always someone elses future and vice versa.