What you seem to be missing is the fact that the video started only after the confrontation escalated to yelling. He very well may have been walking quietly out before this started, maybe he told the cops to fuck themselves as he was leaving which is perfectly fine to do btw.
The facts stated in the original post sound reasonable enough to me, I’m sure the kid made a comment to the cops and they grabbed him to intimidate him or something, at that point the situation elevated to yelling and the rest is on tape.