Is it legal to strike someone because of words?
No, of course not. I honestly can’t tell if he hit the guy or not. You see him swing, hear something making contact, but there doesn’t appear to be any camera movement that could indicate contact. But...I waasn’t there.
If he did strike the customer, then there could be an assault. I doubt a prosecutor would do anything though. But the clerk does not have the right to physically strike anyone because of words.