My high school swim coach was a former Marine DI. I could see him doing something like this. We did a lot of push-ups in our ground workouts. Not necessarily 400 at once, but sets of 50 or 100 interspersed between chin-ups, running drills, leg lift/holds and other fun stuff.
Nobody complained. We liked our coach. We weren’t forced to be on the swim team. 😏
8 sets of 50 over an hour would allow 5+minutes between each set and that seems utterly reasonable to me. I’m not sure I could do it now—but that’s four decades later. If one guy decided to do all 400 in the space of 10 minutes and was capable, and everyone else decided to emulate him, well, that’s their problem, not the coach’s, so far as I’m concerned.
If they can’t do 8 sets of 50, they can go out for power puff.