Yes. That is theft. These kids were not "riding through the man's flower bed" (which was a bad analogy as flower beds have much more time value than a brick), they were skating on stairs and if he wanted them gone he could have gotten management. He took it into his own hands, and they could've taken it into their own hands to prevent theft.
He wasn't completely in the right...but the kids retorting with violence would definitely get them a 1-way ticket to jail. The guy tried to ask the kids to leave, and you know damn well that video didn't tell us everything, we can't know that the guy wasn't supposed to look after that property, or that he might have known that skateboarders were chronically being told not to skate there.
Here's how it should've went.
Guy: Kids, stop skating.
Punks: Ok sir, we'll find somewhere else.
Or.
Guy: Kids, stop skating
Punks: Sir, we have a right to skate here.
Guy: Ok, either you do have this right...or you don't. And since you don't, i'm going to call the cops and get you little fuckers fined.
He was out of line, yes, but I bet $5 this has happened plenty of times before and there probably were signs saying they couldn't skate there.