I’m going to try and answer seriously. There are two ways of looking at it I think.

One is that you take R=U as real and then reflect on what that means for Squall. There’s a hint through the game that Squall has some kind of destiny laid out for him - he’s the one who’s supposed to murder the sorceress, with a blade to her heart. But Squall has no interest in that and by marrying the future Ultimecia abdicates his “destiny” for the sake of living his own life.

The other is that R=U is meant to be ambiguous and the challenge to the player is to leave some things unexplained. I know this is a contentious reference but I’m reminded of Undertale - one of its plot points is that replaying games is a kind of time travel, and if you complete the happiest ending the game explicitly asks you not to restart. You have to wilfully give up content in some form - like seeing the Genocide Run - if you want to honour the game’s own concept. You could look at the R/U parallels and Griever mystery in a similar way: it’s best not to bother thinking about it because it potentially darkens the romance plot, and makes Squall responsible for Ultimecia’s evil deeds.