What would have happened if he didn't got knocked out ? They would have just gone their separate ways ? Don't think so. it was a fight, they were clearly ready to fight, penn just was stupid enough to start the fight by letting the guy punch him as a taunt and it backfiredLetting someone punch you is not a fight.
It's a very stupid thing to do. But it's not a fight.
Equally stupid is not being capable of making these basic distinctions.