not a flop. If you are set in your stance (he was), you cant get both feet (at the same time) to kick out and up without getting a ton of force in the chest. If you watch closely, Smart's feet go forward then up simultaneously. Flops either start with feet going up then forward at the same time(as one jumps back to start the flop), or one's feet going forward then up in a staggered manner, or (of course) if one's feet aren't set.
He did embellish with a sound and a little arm flailing, but he was hit and going down either way.
TL;DR-- Not a flop. A good, strong defensive play. It is impossible to have your set feet go out than up simultaneously without a ton of force hitting you in the chest. Smart waved his arms as he fell.