This doesn't contradict my point at all, I agree entirely that people work with each other and it's a great way to learn. And obviously people aren't going to achieve what took tens of thousands of person-hours at the highest level in one lifetime on their own. One does need to stand on the shoulders of giants and all that.

But the OP was making a much stronger claim, that it is, in principle, impossible to learn anything on one's own, and that HAS to be wrong, for the reasons I listed.