Keeping him at F rank just to be consistent with the manga's name is getting ridiculous.
It feels like the dude is calling himself F rank out of spite, even though that's not his nature.
It's nothing but one more tired trope in these fantasy/isekai series. Occasionally there's a series where the guild instantly recognises the MC's superiority and he/she gets prompt promotions, maybe after some token missions for the sake of bureaucracy, but nevertheless, the guild isn't blind. But in most cases the guilds operate like standard Japanese workplaces where stiff and strict hierarchy and seniority is all that matters.