I think he meant it would teach them a lesson in what is right and what is wrong. They might have chosen a helpless drunk to pick on and mug, but this time they chose a BJJ god, and they got what was coming to them. The lesson they learned is probably just to be more careful who you mug, to be honest. But they deserved it either way (if this is true).
That if you're going to be muggers, you may as well go whole hog and carry a weapon.
Seriously though, the fact that they didn't even have a knife between them and took a while to decide to mug him tells me they don't do this often and may decide it just isn't worth the risk after taking this beating.