Not to defend the guy too much, but as you pointed out he's one of the most physically fit guys in the world, is it so hard to believe that even if he didn't swing with all his might (which many parents do when spanking/corporal punishment styles) he would leave marks like he did?
Not saying what he did is right or wrong for that matter, but I can easily see a guy with his strength hitting his kid in an attempt at discipline and leaving marks like that. Then again I can also see a guy with that amount of testosterone and built up insanity (let's be honest to make it in the pros you have to be some level of nuts) that he could snap and lash out at someone near him for almost no reason.