The quarterfinal bowls will be selected based on agreements they have with the bowls, which mostly breaks down as you said.
However, the Sugar Bowl is a joint SEC/Big 12 bowl. The Fiesta Bowl is not a Big 12 bowl, it’s just a leftover.
So everyone is assuming the SEC champ will go to the Sugar Bowl. Could you create a scenario where the Big 12 champ is above the SEC champ? Maybe. They might still pick the SEC champ, I’m not sure how that would work.
I know this is the way McMurphy presented it a few weeks ago, but I’m not sure why he acted like the Sugar Bowl is only an SEC bowl game. I’m sure BYU fans would prefer the Fiesta Bowl anyway.
Anyway, I’m sure that’s how it will play out, so this is all just theoretical.