it isn't that the MC gets smarter in a group, it's that he managed to find a group of people dumber than him. It's hard to imagine such a thing possible but there it is.
Just because he was weak doesn't mean he isn't experienced. Granted, experience means nothing if you can't back it up, but he has been doing this for a while, surrounded by people even more experienced than him
Every one of you complaining about intellect still? Saitama is scratching his bald head wondering why you put so much stock in it when you could just 'serious punch' your problems?