What he is saying is that Taysom is a great QB but under Ty he didn't look as good. He didn't look as good from a scheme standpoint, not talent. We all thought it was a talent issue and that he dropped off but seeing how poorly Mangum has looked it's certainly changed the way others are looking back at Taysom's senior season.
There's no denying that Taysom dropped off performance wise under Ty, and the same applies to Mangum. In math terms, you identify the common denominator and can reasonably point to the OC as the issue.