His hand touched the rim....it makes his fingers move. You can see it on the close-up angle from in front of the basket.
That said, I think people just overlook LeBron's dunk because he's flipping LeBron and he "only" jumped over a 5'10" ish guy. I wanna see anybody who says "Yeah but Lucas was short" go out and jump over somebody about 8-10 inches shorter than them. I bet 95% of the ppl can't do it. And that's before you factor in LeBron did it while catching a ball and dunking it.