Ben McLemore rotates to defend the rim, because that’s his defensive assignment when he sees someone driving through the paint. The Kings guard even jumps to contest the shot, really fulfilling his responsibility.
Then, you can see the moment McLemore realizes that’s Russell Westbrook charging right at him, and Westbrook is coming hard. That’s when McLemore decides “nope.”
McLemore’s mid-air spin to avoid Westbrook was probably more impressive than anything he could have done to stop the dunk, anyway.