Seen this a couple times. For context, this is an Infant Swim Rescue (ISR) course, probably towards the end of a second or third series of courses. The goal is to teach babies/infants/toddlers to instinctually float if they fall in water and learn to swim towards stairs/edge and cry out while floating. The training culminates into multiple classes where the kid is introduced to the water in full heavy winter cloths, and then "pushed" in a number of times to test the reflex. We did it for our kid starting as a baby cause we have a pool and large pond on our property. It segway'd wonderfully into her learning to swim and at late 2 early 3 years old she's fully mobile in the water and can propel herself from wall to wall with a back swim and is able to do a passible doggy paddle and goes underwater for an underwater swim all on her own. It's pretty amazing, we've never once had her in any type of flotation device (cept on a boat ect) while swimming. She even lets me grab her and take her to the bottom of the deep end in our pool where we wave at each other a few times before coming up. She's also jumping off the diving board and swimming to the side all on her own.
Disclaimer, there are a number of kids who just can't handle it (usually ones that start at 2+). I've seen it myself with kids that go before/after ours. Luckily ours took to it right away and didn't fight it.
Those people recording talking about getting the guys plate etc. The dude that knocked both of them out was being assaulted and/or intimidated by both of them. He did nothing wrong in my opinion.