Interesting to see the doom recreation. The commentary in the subtitles is fascinating.

MPL is a weak copyleft license.

If they make changes to your files then they have to share their changes to those files with a reciprocal license.

It has no impact on the licencing of the rest of their project.

I understand proprietary licenses and the business models they support, I also understand open source licenses and the business models they support.

If they they published paid binaries and free source code I would support them (morally), or if they published free binaries and free source code and ran a patreon I would support them (morally).

But to fork GPL code and hold the derived source ransom? Not cool.