You're right. I said "for any interest" but obviously, like all things attention there's a power law distribution. Many Windows programmers, or anything else that had massive interest. But if you were a Unix kernel nerd, there weren't many Unix kernel nerds around. If you were a Perl programmer, there weren't that many Perl programmers.

But even given the prevalence of Windows development, it was still rare to encounter another programmer socially unless you lived in the occasional city where they were common. My point is that nerds finding nerds was a much rarer thing back then.

I'm not sure the OP was making this claim exactly -- I think they just meant for any particular interest (like running Plan 9 on some old hardware) there were probably only a few hundred people around who shared that particular interest.

Could be. But any particular interest is def not true (windows programming), so more like some niche interest. But this is the definition of niche interest, isnt it.

I can count on one hand the number of people I’ve met that did windows programming, both before or after 2005.

Computer programming was a nationally available subject in secondary schools in the UK in the mid '70s.

I co-wrote a noughts and crosses program to run on a Busicom in 1973; and continued programming at university ('74-'77).

See, inter alia, https://en.wikipedia.org/wiki/Cesil, https://en.wikipedia.org/wiki/Busicom.