this is called the bunching effect, and it's slightly misstated here. if we got to pick the 6 best regardless of order, they would be better on average, yes. this would include all the times that UTG got AA and MP2 had 84o, for example. But UTG would VP$IP, so this situation wouldn't really translate to 6max. we just want to know the 6 hands that are left after UTG-MP1 all fold. This isn't quite the same, from a combinatorical standpoint, as picking the 6 best hands from a group of 10.
if we if we only look at situations in full ring where the first 4 positions fold, then the fact that supposedly better cards remain in the deck has a very minimal effect on average hand strength.
I wish I could link you, but if you search here and in the archives for "bunching" it has been discussed a lot. It's also discussed in one of Ed's magazine articles on blind play shorthanded, which I will link soon.
good intuition though, getting that concept.
EDIT:
Ed Miller on preflop play shorthanded