He has proven himself with his whole body of work not with just as the Lions defensive coordinator. He has had very productive secondaries, and he has done good work.
Glenn is not my pick at all for a head coach but if he is hired then he deserves the support. That's from FOX and then you get this from sportsnaut.com (never heard of them before) they rate the defense at number 5.
Detroit Lions defensive coordinator Aaron Glenn delivered one of the best-called games we’ve seen this season. Dealing with a severely short-handed pass rush, Glenn took the risk of challenging Sam Darnold with an everlasting blitz that put a lot of trust in the Lions’ cornerbacks in man coverage. Amik Robertson had the game of his life versus Justin Jefferson and linebacker Alex Anzalone’s first game back showcased his integral value to this unit’s success. The Terrion Arnold injury is a concern, but he indicated after the game he intends to play in the Divisional Round. While we are still a bit suspect of the Lions’ pass rush and how often this aggressive playing style can work, Glenn’s defensive clinic and the first-round bye restore a ton of confidence in the Lions defense moving forward.
So, there are two different ways of evaluating. Fox uses numbers and stats which is one way, but it doesn't tell everything. Sportsnaut.com uses more real-world examples when rating. Both ways work and both tell a story.
https://sportsnaut.com/nfl-defense-rankings/