He can't process what just happened, but assumed she was weak enough to punch after seeing he tear through his barbed fence with her bare hands? Looks like Katou's bullying days are over.
I was half-expecting Shirosaki to join in. I'm still kinda expecting her to take advantage of this situation to punish the main character. I don't think she wants to break him the way Katou does though, since he'd be physically unable to bully her anymore.