I think what's happening here is: AI. I doubt his videos are being manually reviewed by anyone at Google. Some dubious algorithm is just deciding to demonetize everything.

It may also be that Google has just decided that advertisers don't get any ROI on Linux videos. Many GNU/Linux users may be smart enough to use pi-hole or other ad blockers anyway.

https://www.youtube.com/watch?v=3bgp98X2fGg