Could those who think that the Nebraska game might be a watershed moment be wrong? Sure, but you have to understand that the things you mention (like Nebraska's historical mediocrity) are of no value in dismissing a watershed moment or a significant turnaround.
Team history is of no value in predicting a significant turnaround. No one predicted that Northwestern would finish second in the conference last season after five straight losing seasons (not conference - overall). I'm pretty sure you wouldn't have found anyone predicting the Gophers would finish 4th in the conference in 2016-17 after an 8 win season.
I remember after the great football season in 2019 someone posted here that he thought the Georgia Southern game was the watershed moment of that season. I happened to be at that game and I agreed with him. The Gophers looked dead in the water. They were down by 4 with about 3 and a half minutes left and facing a 4th and 8 at their own 27 yard line. They ended up going the length of the field (all on passes) and scoring a touchdown to win by 3. Georgia Southern was a mediocre team but that end-of-game sequence gave Tanner and the receivers confidence. Up to that point they had not been the dynamic passing team that they ended up being that year. The next weekend at Purdue, Tanner had the game of his life (21 of 22 for 396 yards).
By the way, just as I typed the above message, Nebraska put the finishing touches on their win against Michigan State.