@icam98
Nope, this guy worked at the guild reception , he should know what he or others are capable of. He destroyed that slime bag and the wall behind it(it should have been clear for him how strong he is knowing that he worked at the guild), he also destroyed the training ground(with his fist nonetheless) when he fought with the female knight(even a normal person who isn't even a little knowledgeable about adventurers would think thats not a feat an f rank could accomplish) .
I get that this is the premise of the story but by no means this is ''normal''. I mean it would have been more believable if he overestimated his own power after he was trained by the legendary S ranks or even more believeable if he knew he got really powerful but didn't know were exactly he stood in ranks.