If Github has 21 million active users, Wanstrath is right that current estimates of the size of the developer population must be far too low... Are we under-counting China, for example, given its firewalls? India continues to crank out developers at an astonishing rate. Meanwhile Africa is set for crazy growth too... You certainly can't just count computer science graduates or software industry employees anymore. These days you can't even be an astronomer without learning code, and that's going to be true of all scientific disciplines.
The analyst attributes the increasing number of developers to "the availability, accessibility and affordability of tools and learning," adding "It's pretty amazing to think that GitHub hit 5 million users in 2012, and is now at 20 million." As for the total number of all developers, he offers his own estimate at the end of the essay. "My wild assed guess would be more like 35 million."