Bottas was running on the same day, Mazepin was faster than Bottas on C5 compound. Even if there is a theoretical 0.8-1.0s between them, Bottas is so used to that car by now and Mazepin just had his 1st run in it and he is already on pace. That car is ridiculously easy to drive.
Well, it's obvious, a couple of days of testing against years of experience, Bottas helped develop 2019 car, raced it, qualified in it, I'm pretty sure he is much more used to it than a rookie. I reiterate, even if Mazepin 0.5-0.7s slower on average it's still very impressive.