From what I've read, the information they're gathering already exists and can be gathered by the server (browser type, user, etc.) with an added layer of encryption to ensure that information isn't tampered with which is easily spoofed today. Of course, this approach doesn't stop folks from tampering with the web browser directly to inject whatever information (outside of maybe what browser they're using since that'll be tied to the key) they want into the payload but that makes closed-source web browsers substantially more trustworthy (aka not Firefox) to site owners.
If this does gain mass market adoption, then yeah, I suspect it will force users to use proprietary web browsers (google chrome, edge, etc.). Which is a step in the direction that Google wants.
I imagine that ad providers (Google) can also start throwing their weight to force mass adoption by de-monetizing non-compliant browsers, which may pressure site owners to not serve non-compliant browsers.