I've read through this thread and I haven't seen a simple summary of the situation as it stands with respect to copying and the GPL violation. Here's what I believe to have happened:

1. ChessBase began selling the Fat Fritz 2 chess engine.

2. It came to the attention of Stockfish developers that FF2 is a Stockfish derivative "with a few lines of code modification". [1]

3. Stockfish notified ChessBase of the violation and demanded that they comply with the terms of the GPL. ChessBase responded by releasing the source code of Fat Fritz 2, but not the neural net weights created for use and distributed with FF2. [1]

4. Key Stockfish developers regard the distribution of FF2 without the neural net weights used in creating the work to be a violation of their rights under the GPL. ChessBase (apparently) disagree - I don't see a press release on their website to confirm their take on this.

This question appears to hinge on whether the weights constitute part of a derived work under the GPL. This may depend on whether the weights are "output" from running a covered program. Under the GPL 3, the output of a program is only covered only if, given its content, it constitutes a covered work (i.e. a part of a GPL 3 covered program).

The exact technical details still aren't clear to me. If ChessBase is willing to release any software that actually ships with FF2 (e.g. the NNUE file) under the GPL, I think that would cure their violation even if they don't release the code used to produce the NNUE file. It's not clear to me whether or not they have been willing to do this (I'm not totally sure what's meant by "weights").

5. Regardless of this, Stockfish developers likely have the right to invoke the nuclear option (license termination). Even though ChessBase have ceased their license violations, a GPL 3 copyright holder may terminate your license under 2 conditions:

* If you have been notified of a GPL 3 license violation by the same copyright holder (for any work) before, and the copyright holder notifies you of the violation within 60 days after you cease to violate the terms of the license.

* If you do not cure your violation of the GPL within 30 days of notification, the copyright owner may terminate your license regardless of whether you have previously been notified.

6. There are two ways Stockfish potentially wins this lawsuit. Either

(1) they argue that violation of the GPL license is ongoing, and thus ChessBase have not cured their violation of the license within 30 days and therefore they are within their rights to terminate. This would hinge on the licensing status of the neural net weights, as discussed above.

(2) They argue that at least one Stockfish copyright holder involved in the lawsuit has notified ChessBase of a violation of the GPL 3 before, and thus has the unconditional right to terminate the license because ChessBase was notified within 60 days. This seems plausible enough given ChessBases's previous use of their code [2] but I haven't found any statement that outright says that they have previously notified ChessBase of a violation of their GPL 3 licensed work.

[1] https://stockfishchess.org/blog/2021/statement-on-fat-fritz-...

[2] https://lichess.org/blog/YCvy7xMAACIA8007/fat-fritz-2-is-a-r...