You seem to conflate “GitHub users” with “[people] who use git/source code control”. That is almost the opposite of a point that I wanted to make. It is obvious that GitHub would have many popular web projects, but if one could sum up git projects instead, this would probably not be the case.

I'm curious, what communities and places do you know most of these developers from? And what sort of platforms do they work with?

Most programmers I know professionally and personally are working within variations on the theme of civil engineering, logistics, finance or telecom. Sure some of them occasionally write tools that are accessed via a web browser, but non of them would probably classify themselves as "web developers".

I see. This is similar to my experience. Most software is made for people to use at work, usually semi-custom. Most software also doesn't depend on low latency or high graphics throughout, so most applications, especially new ones, are web applications. Most of the developers I know that do similar work would call themselves "web developers" though that may be required to context. Someone might identify themselves as a web developer to me, a developer in an unrelated domain, but not to a non-developer in the domain.

I think it depends very much where the emphasis of your effort is spent. Are you writing an analysis tool and then putting up a web front end to show the results, or are you writing web front end to interact with an analysis tool.