Some things just don't make sense here. MC thinks he "saves" Nanao from the douchebag who was trying to belittle her thinking he'd easily defeating her. While douchebag only knows from hearsay about the fight against the troll, MC saw it firsthand and should know/ have realized Nanao's combat proficiency. Yet, in his thoughts immediately before the battle starts, MC also belittles Nanao, thinking he'd get an easy win being then completely suprised about Nanao's combat proficiency. Why? For some bullshit reason of "Ok, you can beat monsters, buit how about a fight between humans?" I mean, it's not like you can hone your swordsmanship without ever sparring with someone.
On another note: Seems like MC words towards douchebag were so razor sharp, they even left a hole in douchebag's forehead, LUL.