They have daemons running to manage that and do stuff like add IP addresses when IP forwarding rules are added; https://github.com/GoogleCloudPlatform/compute-image-package...
Edit: sorry didn’t read far enough and have now answered my own question. In VMs. So yea, Amazon could not duplicate without getting people to change AMIs or install some agent. Still seems like an SSH CA is a better option. Curious why google didn’t go that route. Maybe not an option when they built this?