I understand what you’re saying. And I am not a big fan of Garret at all.
I do think, either way you need to have one of the more talented rosters to win.
Even the patriots. Yes Bill Belichek is the best HC ever. But their rosters are always very talented as well. Maybe not the most talented in the league, but consistently top 5. This goes back to their scouting and player acquisition as well. The patriots don’t overpay players. But they are adept at finding talented players who fit their system and can do the jobs asked of them at a high level.
The eagles in 2017. Also had a LOADED roster. Full of top notch talent.
So I think just about every HC needs top end talent to win a championship.
But you’re right. Discrepancies in coaching does make a difference, especially when talent levels of rosters are so close at the top of the league. Rarely in the playoffs are you significantly more talented than the opposition. Usually the difference in talent is small