The problem was many were saying he would never be that guy. As he was, was all he would ever be. A lot of shortsighted people on this topic.
I'll admit I thought Matt's offense would click at some point but I also always thought he would be mediocre at best on defense.
Thomas' defensive improvement is one of the most impressive improvements I've ever seen out of any player. Players often improve their shot, offensive moves, decision making, ball handling, and rebounding.
Thomas improved his defense not just by effort and learning what to do but he's defending in a way he flat out didn't look physically capable of last year.
Eg Burton could make huge improvements on D next year but he clearly already has the physical capability to do so, it's simply a matter of focus, effort, and knowing what to do.