Saturn getting focus in the flashback doesn’t change things. Kizaru is still Luffy’s fight.
This is exactly what happened in WCI.
- BM was the main villain
- BM got a flashback and tons of focus
- Katakuri was Luffy’s fight, BM didn’t even get defeated
Similarly, it makes far more sense for Kizaru to fall and for Saturn to take an L but not actually be defeated this arc.