Seems like people are mis remembering how he actually played, or basing that sort of assessment on one summer league game where he played well.
Sure you would expect some improvement from an truly abysmal offensive rookie year, but nothing's guaranteed coming off a knee injury. And even though he's got great defensive potential that is vastly over stated too. Gobert gets most of the credit for the 2nd half improvement to end last year.
Exum right now is more potential than good. I favor trading for a more proven pg, like Teague.