That's the way I took it. Little Pusher probably isn't used to being caught in the act, let alone called on the carpet immediately by someone in a stern voice of whatever volume. I'll wager he's not an angel in the classroom, either. When he pushes, he probably gets the, "That wasn't nice. How do you think that made Johnny feel when you pushed him? Would you like it if someone pushed you?" If he's like half the kids, he hears, "Blah, blah, blah, blah, blah..." and is thinking, "Sheesh....Get it over with so I can get back to the slide already."

Come on, he KNOWS pushing isn't nice, but he did it anyway. By 5 years old, you should know to not push.
DD went to preschool since she was 2 and hardly any kids were agressive like this. Most were taught by their moms with a firm, "NO!" that pushing was not acceptable, followed by removal from the playground. Take the kid's playground privleges away for a while and explain why and maybe he'll have some incentive to stop pushing. Explain the error of his ways for 90 seconds and let him back on the slide and......well, good luck with that.
I can't blame the mother of the kid who was pushed. I would not take kindly to some bully (and yes, I think the kid is a bully) pushing my child TWICE within a short period of time, once being at the top of a slide, which is very dangerous. A few terse words probably did that kid a world of good. It sounds as if no one ever let him know before, in no uncertain terms, that his behavior was unacceptable. I'll also bet he does not bother that kid again.
I probably wouldn't have yelled since I am not a yeller. But a stern warning and a severe evil eye.......Yeah, he'd have gotten that. Forget waiting for the teacher. I saw it, she didn't. And a warning from the victim's mommy carries more weight, IMHO. BTW, if my DD had been the pusher and some mother took her to task....Well, that's what she gets for being a bully. Bad things happen when you're a bully. I expect other parents to let me know if DD misbehaves.
The bully wasn't damaged for life and may have learned a lesson. The victim saw his mommy stick up for him.

The teachers now have a heads up that they have a pusher on their hands and they need to watch him. When I was a kid, if ANY mother saw us misbehaving, we were fair game for a reprimand. We knew that and it did have an effect on our behavior. Too many kids today know that their parents are of the "my child is an angel and no one had better ever get on to him" and they take full advantage. Now that Little Pusher knows there are eyes watching him, he may straighten up a little. And that's a good thing.