Various versions of Windows have had their source code leaked out in part or almost whole. If Claude produces an exact copy, like LLMs used to do with the fast inverse square root from Doom, Microsoft would have good reason to sue and it'd be on the project to prove that the copyright violation was done by a bot (which makes it legal now).

With the project essentially implementing the entire API method by method, the chances of LLMs repeating some of the leaked source code would be tremendous.

A one-directional fork of ReactOS might be able to make some fast progress for a few people who desperately need certain programs to work, but I don't think the project will benefit from LLMs.