But this is about SSL certificates. Google may account for say half of web traffic, but there are billions of other servers that account for the other half, and it has absolutely no care what web server or ACME client they are running or much else. It is concerned about the client experience and how it trusts certificates.

Google already has its own CA that is used for its own systems as well as to issue certificates for GCP customers. They don't interact with Lets Encrypt or any other external CA as far as I know for their own services.