GPL

I took GNU/Chess. Heavily modified the AI, using neural nets, and then made it into a paid online service. Say integrating it with some web chat or discord channel. How would Stallman Nazi will ever prove that AI was modified, if the source code runs on server side?

Even hackers leaking source code wont prove anything, since hackers are not a trusted source and can modify everything they leak. That is why you should never trust the scums like Assange and Snowden, who heavily redact what their leaks, provided by Russian and Chinese hackers.

This is actually legal because GNU Chess is not under the AGPL. Regardless, your thing will fail due to lichess which is foss.