Did you see the slow-mo replay of that? That was no punch... it was far closer to a "hollywood slap," more along the lines of the Three Stooges. It has fakery written all over it... remember.. with the Oscars' ratings long in the tank and getting worse with each passing year, the producers desperately needed some form of "drama" to spike the viewership.
It was a slap with most of Smith’s four fingers. You could hear it. Maybe not a lot of physical pain for Rock in it.