A bit uncertain what you mean here. I agree that Kosar & Bledsoe had more raw talent than Brady but it takes more than raw talent to succeed in the NFL as a QB. It requires a rare combination of talent, vision, spatial recognition, quick information processing and the drive to study for hours the cutups supplied to the QB.
I cannot say for sure that Belichick made Brady better but given that most agree that Belichick is an excellent coach and that Brady was not an outstanding collegiate QB, it seems likely that Belichick's coaching made Brady a better QB. Not many QBs become better in the pros than they were in college (Brady & Josh Allen and maybe one or two others excepted.)