> If you change your service’s API, just updating all of the other clients of your service in the repo is not sufficient to skirt the problem of different clients expecting different APIs.

Most Google APIs are written in protobuf, which is backwards compatible over the wire. This makes most changes painless, although caveat emptor as things can still get messy as they do in the real world. With something like a Java interface, it doesn't really matter if those APIs change because all the code is built and released together at HEAD.