He wasn't going down at the time. He was trying to keep from going down by using a stiff arm that to many initially looked like a punch
A stiff-arm is a push away, not a slap up side the head. I think it was a good no-call, but it was no stiff-arm. The next ref may call it.
I watched it several times. In Fournette's defense it looks like the would be tackler is grabbing Fournette's face mask
None needed, been on both ends of punches, that wasn't one. My father in law calls high school and college football games and he said he wouldn't have thrown the flag on that.