I think if he did actually assault a border guard, he would never have been let go (edit: so quickly). The surface facts that we know and can believe (detained and released) smack of punishment for some small slight. His punishment (beating, citing for a felony a being immediately released) is completely consistent with many other cases of people saying the wrong thing.

Make no mistake, when you really do cross the actual line with a cop/border guard, you will pay with all the force of the law (and then some).