Does anyone know why Valve forked the Wine rather than joining them?
"Modifications to Wine are submitted upstream if they're compatible with the goals and requirements of the larger Wine project; as a result, Wine users have been benefiting from parts of this work for over a year now. The rest is available as part of our source code repository for Proton and its modules."
Wine has very tight/strict requirements for what can merged into its codebase, it's why DXVK has had to remain a standalone project because it doesn't live up to Wine's code/goal requirements.
"Get all windows stuff to run perfectly in Linux" sounds like a pretty simple goal to me.
The main Wine devs knew but were probably asked not to tell anyone, and apart from them I don't think many people scrutinize so much the personal details of all the people who submit pull requests in all the open source projects.