Pereira has put on a shit ton of hours on the mat with Glover to stop himself from getting submitted like he did against Ottoni. It's an obvious hole, but as his fight with Jan showed, he's more defensively sound than your average striker crossing over.
Of course good boxers could win MMA fights, they can still apply their specialty, not unlike your IBJJF competitors/D-1 wrestlers/Olympic athletes, but that doesn't mean they wouldn't also take some pretty embarrassing losses to guys who don't play their game. It's normal.