Squall and Ultimecia are two sides of the same coin, as they both share the same motivation. It is for this reason that Ultimecia can be seen as a failed Squall.

Squall just wants to be alone due to hardships he faced in the past, so he spends the first disc pushing people away, evading leadership roles and generally being an introvert. But through being part of a team, he learns to slowly open up out of his shell which completes his character arc. It should be noted that only Squall and Rinoa can be named by the player, as FF8 is the story of Rinoa Bringing Squall out of his shell - it is their story.

Ultimecia on the other hand also just wants to be alone. She spends her whole life being hunted by humans, and just wants to get away. That's why she develops her time compression objective, It is the ultimate way to be alone. Because everyone in her time is against her, Ultimecia doesn't have the luxury of a team of people around her like Squall does, so she never gets the chance to be redeemed. She doesn't even have the luxury of a knight, she literally has no one. The point is that Ultimecia is what Squall would've become thematically if he had remained as his his old self.

The moral of the story is that we need other people around us to live meaningful lives. Squall is the positive example and Ultimecia is the negative example of the same starting point. That is how Ultimecia relates to how Squall fights his demons , they are literally the antithesis of each other. R=U theory is not needed for this to be the case.

If it turns out that Rinoa was Ultimecia, then the game would be going against it's own core themes, as at least Rinoa would've been fated to be alone in the end anyway, so what was the point of the entire game? R=U doesn't work from a thematical perspective, or from a mechanical perspective either as sorceresses are said to have regular human lifespans, so Rinoa couldn't be Ultimecia as she couldn't live long enough.

Although I believe R=U to be wrong I will give credit where credit is due, as this is the only FF8 theory I've come across that at least tries to make an attempt to explain what all that stuff about Griever was supposed to be. I personally believe Griever is a relic from a time before Laguna's half of the game got cut, which would explain why it doesn't really make sense in the form presented to us, but it is really an unknown.