This is patently untrue.
1. Jets were ranked 6th in the NFL in total defense last year, despite a pretty empty cupboard
2. He was not making the personnel decisions and admitted that he 'allowed' it to happen
3. Bill Belechik was not very successful in his HC gig either, he learned and evolved
4. Rex Ryan was the Def Coordinator for the Ravens, who were pretty good as you recall
Look, no one disputes how good BB is as a HC. Does not mean Rex sucks or even that he might be elite, time will tell, he started strong and then lost control with what as we have seen, was a pretty dysfunctional front office in NY.