As far as I can tell there is no public source code beyond a few tools they use, and many those repos have no license. So while his actions are shitty, he didn’t legally give them code. From what it sounds like, he gave them a compiled asset with access to his source code, then revoked access to his source code repository, so they decompiled the asset and he’s suing them for that. Which he may have legal grounds to do, because I’m guessing this was occurring in the “let’s all make a mod together!” and there likely isn’t a formal contract signed that says “anything you contribute to the project is ours.”

Unfortunately, more projects should get this "in writing". GNU projects prefer copyright assignment, but (I think) many of them also accept just a clear licensing statement ("This is mine and no one else's; I give it under the project license." but in legalese).

If there's no clear statement, a judge many very well issue a preliminary injunction against the project distribution, just so they have time to gather and review evidence, even if the actions of the contributor are later found to show intent. With a dated statement containing the contributor identifier(s), a judge is much more likely to accept an initial motion to dismiss (even with prejudice).

He's either got to have trademarked the code or have to prove damages