Unlikely that everybody is on PEDs when the sport allows 33+ year old individuals to be champions and dominate.
EDIT: Original post has some flaws, so here's to phrasing my thoughts better: i don't think there is anything to suggest that everybody is on steroids in the UFC.
If the level of athleticism was so insane that PEDs are necessary for everyone, than we would not have champions/top contenders that are 35-39+ years old.
Because if athleticism (which is where PEDs help the most) was the main factor/so important for winning in MMA-fights, then champions/top contenders would be no older than 32-33 years, which is where athletes in sports that require solely explosivity and/or anerobic capacity are out of their prime. (if not years earlier).