I wouldn't say he made Cooper and Cobb better. Cobb had some of his best years with Aaron Rodgers. Cooper was a 1,000-yard receiver before becoming a Cowboy. Gallup, Schultz and Jarwin are washes because we really don't have a scenario for comparison with them. I think we will find out more about Schultz now that he's fled to Houston (if memory serves me correctly).
Be that as it may, I think it's a stretch to imply he made Dez, Witten and Coop better. Besides, who's to say they didn't make Dak better? Nevertheless, Frog's statement made me take note.