Looking back, I don't know that their D was all that elite at the end. In 2022 they played one ranked team - Ohio State, who hung 31 on them in the first half, 45 through 3 quarters. Yes, they were 17th in ppg/game. But that includes:
Illinois State (FCS, shutout),
New Mexico State (82nd in ppg, gave up 7),
Northwestern (128th in ppg, gave up 7),
Iowa (123rd in ppg, gave up 17 with 7 coming from a pick-6),
Nebraska (102nd in ppg, gave up 14),
Illinois (95th in ppg, gave up 34 and lost),
Michigan State (92nd in ppg, gave up 34 and lost)
Washington State (80th in ppg, gave up 17)
Purdue (75th in ppg, gave up 24),
Maryland (67th in ppg, gave up 10),
Minnesota (66th in ppg, gave up 23)
Hell, not only was Ohio State the only ranked school they faced, it was the only school in the top half of ppg that they faced.
Leonhard is also partially responsible for the lack of talent and depth on the current team. He's partially responsible for anybody recruited before 2023, which should be most of the 2--deep.