1. I think it's fair to say that the quality of their play was last season was easily in the same ball park as it was under Corbin. Millsap was used differently, sure, but much of that is due to roster differences. The Jazz were better under Corbin than the Hawks were last season.
2. Even if they did improve, it's awfully tough to disentangle the effect of the coaching change from improvements made by the player in the offseason independent of coaching. Millsap in particular has added little things to his game every offseason.
I've just noticed this sentiment a lot on this board, and don't think it's accurate. I get that people want to believe that the young players remaining on the team was held back by Corbin, and that their elite talent will finally present itself now that Ty is gone, but I think it's just wishful thinking. The Jazz still need a lot more talent.