I'm not familiar with the situation, but my guess would be that ChromeOS has created its own Wayland compositor with a bunch of private protocol extensions that

I'm not familiar with the situation, but my guess would be that ChromeOS has created its own Wayland compositor with a bunch of private protocol extensions that Chrome on ChromeOS uses to get stuff done. But for some reason or another, those protocol extensions are not suitable for submitting as new public extensions for other compositors to implement. Or they are suitable for submission, but the official extension roadmap already has different extensions in the works (but not yet complete) that will do the same thing.