Just looking at Glenn's history as their DC is unimpressive. I think his leadership is what gets him his popularity as a Head Coach candidate, not his Xs and Os.
Lions Defensive Ranks with Aaron Glenn:
2021: 31st in scoring, 29th in total yards.
2022: 28th in scoring, 32nd in total yards.
2023: 23rd in scoring, 19th in total yards.
2024: 7th in scoring, 20th in total yards.
The Secondary, which is supposed to be his specialty, ranked 30th in 2024, 27th in 2023, 30th in 2022, and 24th in 2021.
The Lions have spent 11 of their last 16 draft picks in the first three rounds on defense, as well as signing numerous free agents.
I'm not saying Aaron Glenn can't be a fantastic Head Coach, just pointing out his defenses are a red flag.