Well, it's not Claude, it's GitHub Copilot (which happens to be owned by none other than... guess who): https://github.com/reactos/reactos/pull/8516

But, if any such model got fed with leaked code, then how is this a specific open-source project's problem and not of all projects (either open-source or private) that got to ever use that model?

Then, (having thought this just now) how can an argument relying on (legally) undisclosed information be used against anything public? Isn't the onus on the party having the undisclosed information to prove that it preceded the public one? How can that precedence be trusted by an independent judging party if the undisclosed information (source-code and systems managing that source code) is and always has been in the hands of the accusing (thus biased) party?