im not sure +6 is considered normal, at least from what i remember it was always referred to as absolute minimum to win a grand tour - so obviously anything below that is "weak" when talking about gc which is what the doctor is talking about, the group of other pre/con-tenders on that stage was weak and you dont even need the numbers to support that - when Formolo pulled out he stayed in that group untill the finish, they even caught Carapaz ...on that specific day the rest of gc was underperforming, while pogacar maintained what would be expected from him
on ventoux stage, his level dropped off a bit while others were stronger, again supported by the overall times, especially last 6km from vingegaard were extremely fast
im pretty sure all grand tour winners and contenders are capable of 6+ performances, how often and how much above 6 depends on quality (quality of athlete and quality of PEDs) ranging from armstrong/pantani(6,5+) levels through pog (on good day 6,5), contador somewhere around same ability, bernal must be pretty high as well
on the other end TGH was the luckiest grand tour winner since Hesjedal, the level of competition was so low he probably only scratched 6 watts even on good days