The author of that article mentions two coaches, who were working for the Chiefs and the Buccaneers, one who has been passed over for 7 head coach roles this season and the other who can't even get an interview and he cannot see why, as they are both better qualified and experienced than those getting the jobs. There is still a lot wrong with the NFL in this regard.
I think the key is which franchises you talk about. If you have the chance to work with Mahomes or Brady every day it will be a lot more meaningful than getting a job at Cincinnati or Jacksonville. I sincerely doubt Bieniemy really wants to leave the Chiefs. He's got an excellent job in the position he's in. He probably shows up to the interviews and makes it pretty clear that he has extremely high salary demands because he loves his current job. I genuinely believe he really values working with Mahomes, Kelce and Hill in the most explosive offense in the NFL and who wouldn't? Also, to be #1 in line when the ageing and (let's be frank) not very healthy-looking head coach probably won't have more than a few years left in the game is not so bad. Bieniemy could easily land the big job at the Chiefs if he sticks around.
So I think this is mostly a stick being used to beat NFL franchises with when in all likelihood Bieniemy had a lukewarm interest in the teams that gave him interviews. One of the teams that rejected or got turned down by him hired a less qualified black coach one branch below Bieniemy in the Baltimore Ravens' coaching tree. Teams that would've been of serious interest to him because of the assets involved didn't hire new coaches this spring (Chicago, Dallas etc) so he probably feels better off where he's at.
There are definitely some opportunity barriers at a base entry level so it's a fair point but in this specific case let's not pretend that being in consecutive Super Bowls as offensive coordinator in the most stable organization in the league is a job to be scoffed at above dealing with players who couldn't tackle a lamp post or hit a garbage can with his throws
Think of it as going from van Dijk to a random Championship hoofer. Even some top college coaches aren't interested in the bottom rung of the NFL because it's very tough to turn those projects around.