You don’t need to cut the internet in half to limit China to routing IP adresses that are allocated to China.

IIRC, Google can try to dictate a BGP policy that says not to accept any routes that goes through China. However, without any verification checks (via cryptography), an entity can lie about the path that they are advertising.

No, the peers just outside of China can choose to reject advertisements of Google by China. Google can’t do much.

There is already a wall. This may be a case of something leaking through it.