You and I know that some of his best wins came against G5 teams yes. However nationally, that is not how BYU came into relevancy.
As a BYU fan since 1974, my best memories have been when BYU has beaten respected P5 programs, not the plethora of wins against Wyoming, New Mexico and UNLV.
It's not about what his best wins were. It's about beating SMU and Missouri and Michigan and Penn St and Notre Dame and Miami and Texas and Oklahoma and UCLA and Washington and Texas A & M etc. etc. Their final records when we beat them are also irrelevant.
The simple fact is, nationally people notice and respect you more for beating a 6-6 Texas team than they do for beating a 9-3 Fresno St team. That is always how college football has worked whether fair or not.