Is it just because they played a bunch of non-P5s? Since 1980, here are the largest average margins of victory for non-P5 programs, who likewise had really "soft schedules" per CB conventional wisdom:

The 1984 national championship team (MOV 21.0 ppg) is 38th on this list.  Sure, the 2020 schedule was easy, but it wasn't like they struggling to win games against these teams.  Nearly all of them were over well before halftime.  For those who haven't emerged from the stone age, every credible analytics-based ranking system is going to have MOV as a major component in their rankings.

If simply having an easy SOS was all that was needed to top this list, then there'd be a lot more teams with a MOV at or above 28 ppg, right?

Now, let's take a look at who was on that 2020 roster:

Sure, if BYU played 6 P5 programs in 2020 and COVID wasn't a thing BYU probably wouldn't have gone 11-1, but that doesn't mean they weren't a very good team.  IMO, most fans dogging 2020 are simply people unbalanced by their dislike of Kalani's performance as a HC and concoct a bunch of poorly crafted arguments against that team.