At some point in time these authors just gotta come out and say that the MC is mentally challenged because idc how apathetic a mc is or common sense deprived at some point you gotta start asking questions and looking things up.
It's the fantasy hillbilly trope; basically, a person who lives in the mountain or countryside comes to the city and has none of the common sense of the city folk regarding fantasy monsters because back in the boonies you just shoo them away.
I get the author wants to show off the protagonist as the #1 strongest, but come the fuck on. Taking out a large number of dangerous ants solo by just smacking them is already an impressive feat. They even point out the potential danger of how they can easily overwhelm someone, so there’s no reason to add this stupid power-scaling bullshit.
Personally, I don't doubt that 10 ants could destroy a city; if they can't be stopped, and they can rip metal and concrete apart then it's really just a question of the time (weeks? months? it's definitely less impressive once you state it'll take a long time XD) it would take and they would have to abandon the city for safety reasons.
I assume that a single ant could be stopped by a party of adventurers, but a group of ants couldn't be stopped by scaling the number of adventurers. It's a problem of space; the ants are small and can cooperate and concentrate their power in a small area. You couldn't fit enough of the highest levels of adventurers they have in the small space in which 10 ants operate to take them down. If they can get stronger adventurers, like the MC, then it's no longer a problem.
Or maybe what they mean by that is that the ants are so resistant that whatever they need to do to beat them would wipe out a city (e.g. a nuclear bomb). XD
But yeah, I also found the statement hilarious. We're missing some context that would make the statement reasonable and we can only speculate (as I've done) as to how, and in what time frame, 10 ants can destroy a city.