Only the parts written by LLMs would be public domain. Everything else would still be copyrighted by Microsoft. See https://fsfe.org/news/2026/news-20260825-01.html

And since there's no easy way for outsiders to determine which part is what, it would be a minefield for anyone trying to use the leaked source code. ReactOS devs would have to stay far, far away to avoid giving MS an easy way to sink the project