Enthusiasts always exists and there is a low capital requirement. Just like you get anonymous art, people volunteering for all sorts of things - not everyone does everything for money or even recognition.

And even open source typically has some licenses language associated.

Job pays them really well and/or they are very well off already, so a minority have the time and passion to try and make something on the side. Even then, the biggest and highest value OSS have full time maintainers and offer pay for that. Are there really any Linuxes out there that were purely maintained by free labor?

Alternatively, students who want portfolio pieces. They have an incentive to stand out in a job market and make money as an endgoal.

There's lots and lots of OSS licenses out there, some more lenient than others.

I personally know quite a few OSS devs that don't care if individuals and small shops use their stuff in whatever way they want to, but obviously it's much different when a nameless megacorp does it instead.

There are some open-source games and fan movies, but they come nowhere near the quality and quantity of the commercial titles. For every tux raacer, there are 5 Mario karts

It would seem that the "almost all" in their comment already does.