Well, he was offered $2.5M to be associate head coach and DC at LSU. That's demand. If you don't think he could have gotten P5 jobs I am not sure what to say.
You need a list of top coordinators that made a jump to a national championship contending program as a HC? How about everybody's favorite coach this week, Lincoln Riley? Same exact thing. In his 30s, OU. Kirby Smart, who has UGA #1 as I type this too. No HC experience. Right to UGA. So literally the hottest coach in America and the HC of the #1 team. Same for both. Dabo was promoted from assistant at Clemson (he wasn't even a coordinator iirc) to the big job. He won it all. So that's maybe 3 of the 4 top guys (Saban) who fit the bill.
There is a reason you can't just look at a piece of paper/resume and make a hire. Or being an AD would be easy. This guy has been crushing it everywhere, has a HC temperament, and was going to get a big gig within the next 24 months for sure.