No way to know for sure. Yes Belichick and company won 11 games in 2008 without him. With Garrapolo/Brisett they went 4-0. Is that all on Belichick? No. Some absolutely.
Special type of Coaches can do that for a team. In all the years I watched the Shula coached Dolphins, I always knew, that no matter who the next few opponents would be after a loss, it was almost a given that Miami would not lose the following weeks, and same thing happened to teams coached by Landry, Lombardi, among others.
I agree with you that players do have something to do with it, but good coaching goes a very long way, considering a team with good coaching, but average talent will normally be more successful then a team with good talent but average coaching. Consider all the Patriots backs...
Cassel, Hoyer, Mallett, before Garrapolo. Each when give their shot looked like they could be quality starting Qbs, but outside of the Patriots, Cassel and Hoyer proved to be at best OK, and Mallett, not so much. Garrapolo could be the exception, but might also be yet another QB that was made to look better with great coaching.