He was shot in the ear and presumably frazzled and had the presence of mind to recognize an enormous political windfall he could capitalize on rather than just letting the Secret Service usher him away as they clearly wanted to.
Try being shot at, and grazed by a rifle bullet, knowing that you came within a whisper of death from someone armed and specifically hoping to kill you yourself, there with your own blood running down your face, and then still having the presence of mind to stand up and pump your fist in the air. Imagine all this at the age of 78 and with no previous combat experience or with situations where you were being shot at.
No, it would not be easy and far from common to take that in apparent stride so quickly as he did. It was indeed impressive regardless of what you think of Trump the candidate in all other regards. You saying anyone would raise a fist is absurd.
What capability he showed? Knowing you are guarded by tens and hundreds of officers, anyone would raise a fist.