I really take issue with that. I think it is blatantly untrue. I've worked in small towns and I've worked in large metropolises (London, for example). I am not more productive in a metropolis. It's handy to go to meetups occasionally, but I have to say that with the internet I learn more by watching interesting talks and emailing people occasionally than I ever did going to the pub and chatting with super famous people in person.
I mean, I enjoy going to the pub and chatting with people in person and I really miss that about London, but I'm way more productive in my current rural setting.
It is, however, instructive to note that I am a remote worker. As a business, if you want to have people who are local, then it helps tremendously to be able to select from a large pool. That's obvious. As an individual it's much easier to network in person and so it's easier to get a job. But, when you are actually on the job? I really don't see this increase in "productivity" at all.