Pek doesn't get up that high, but I think you are underestimating his role in this dunk.
Look where Barnes took off. You have to consider that Pek is, arguably, the strongest player in the entire NBA. You launch from that distance against a guy that strong (who, btw, pretty much timed his jump perfectly) and chances are you will be on your ass; at the very least get nudged off enough to miss the dunk. Barnes went right through him.
And what made the dunk for me was how he cuffed and the cocked back before he shit on Pek's face. Nice touch. It also had a good build up because you could see it working in progress starting from Barnes pointing the cut to Lee. Helps that it was at Golden State to get the crowd's reaction as well.
Only thing that would have made it better was if he did it in from of Minnesota's bench. However Golden State's bench was hilarious so they did their job.