There have been some excellent coaches that never won a SB or pre-merge NFL championships.
There have been 31 different coaches that have won SBs. Weeb Ewbank, Don McCaffery, Gary Kubiak and others have won SBs, so does that mean they are great NFL head coaches? All of them?
Not defending Garrett as a great NFL coach, before anyone reads that into this post. He's had chances and didn't "get 'er done". At some point you have to make a change.
But winning a SB doesn't automatically make you a great NFL head coach. Mike Ditka won one, but with Buddy Ryan's 46 defense that allowed a TOTAL of 10 pts in 3 playoff games, and those 10 were in the SB. So Ditka was a better coach than Garrett?
Who knows? We aren't privy to all the details of those teams, to suggest that you have to have a great NFL coach to win a SB isn't borne out by the data.
So if Garrett does win a SB, you HAVE to give him credit for at least some of that win, based on what you know about every SB winning head coach, which isn't all that much for those here..