As I understood it, the previous maintainers (Jens and Thore) just resigned from the project, leaving it without current maintainers, but with the github and other infrastructure still intact. Since the project is open source, it would be natural, if you needed new features, to add them into the existing project, causing new nightly builds to emerge with those features for all supported platforms.
Or have I missed something?
No, you haven't. The whole point of making YAM open source was to allow development to continue. Jens and Thore did a great job with the build environment, ensuring nightly builds are automatically built online at GitHub. It is released under GPL, so I cannot explain why Amigakit are attempting to steal the code (including the few changes I have made since Jens and Thore stopped working on it, claiming them as their own). Why not just contribute to the YAM repository so everyone can benefit and contribute?
It is like me reverting AmiSSL to closed source, after I took over all development - it would not have been the right thing to do.