Besides choking them out, I'm not sure what else he could've done. You can try to convince them without triggering their egos by asking about their swimming ability and ocean experience, but who has time for that? It's not really the place for a socratic dialogue.If I'm following the dialogue correctly, these guys had already been told by another LG not to paddle out. If you've already been warned once and you disregarded the person who is responsible for your safety, it's naturally going the change the approach from the LG.
If that LG has to affect a rescue on those guys and increase his exposure to life threatening conditions because of their stupidity, he's justified in being terse. If for no other reason than to get their attention and encourage them to grasp the seriousness of the situation.
Also it's fun watching kooks get chewed. We should encourage this.