Wesley Matthews was a slam dunk champion too. When I see Williams dunk, I see a kid trying hard to dunk and pulling it off, not Vince Carter. Just because someone can dunk and does at every opportunity does not make them athletic.
Williams is a very fluid player in his footwork and his general movements, but I don't see crazy natural explosiveness nor insanely quick feet. That's not to say he's not been stunning, but I just don't see a naturally explosive player. But he does a good job with what he's been given, but he relies in his intelligence, versatility, and aggressiveness to score more than what Perry Jones does (a magically athletic idiot who can just jump over people when he realizes he isn't late-career Kobe Bryant).