I'm not sure I understand what his misunderstanding is. He was clearly told that the monster he killed, that he acknowledges that he defeated, was something an s-rank party would be needed for.
Yet he still acts like he has no skills and no ability, despite this - if his dream was to be an adventurer, he should be leveraging that win to become an actual adventurer.
Add on to this that the series is pitched as being about a guy with only one skill, but he actually has starting skills from all the warrior types, and has apparently max leveled them - so how does anyone, including he himself, not release his ability?
It's just kind of breaking the believability of the story so far.