Just my opinion here, but the way this MC is written is just plain terrible.
He basically has a brain damage for being so stupid and stubborn, and the guy (ex-sage) arguing with him isn't even trying to explain why he's not just being modest.. People just don't talk that way, giving up half way when making an argument about crucial stuff like that, especially when they know they're in the right..
I feel like the art style is wasted on this crappy writing