Actually, my first thought...or rather, gut feeling...was that it just didn't look right. Something about it just seemed, well, off. Add to that the fact that the punch itself isn't shown, just the aftermath of a supposedly crushing blow, and I'm not too certain this is any more legit than most of the nonsense floating around on the internet.
I've seen fights, by the way, and it is entirely within the realm of possibility to be floored by one punch...seen it happen, heard about it, etc...but this just looked, as I said, "off".