Probably because new weights are the only thing they brought to the table, rest of it was just stockfish so it didn't hurt business interest to reveal generic s

Probably because new weights are the only thing they brought to the table, rest of it was just stockfish so it didn't hurt business interest to reveal generic stockfish source, once people realised Fat Fritz is just stockfish. In terms of GPL violation, I think if you are serious about the subject, you should simply shoot a mail to FSF, I am sure they could get back with something definitive. Personally (and IANAL) I do not believe in this case there is any ML specific nuance. As I understand, GPL demands you reveal source code to the full extant where it can grant its users the ability to recompile the software themselves. To that end, you can't omit any critical source file, because then the program doesn't compile. From this perspective, the model file is no different from any source code file, its absence renders the software un-compilable. Editing the model would still be same as editing source, thus still falls within GPL. However, I don't know what the implication could be, if the model wasn't baked into final executable, and instead was loadable at runtime.