So, customized Google OS, but using the official Docker package? Oh wait. How could there be an official docker package for an OS noone known existed?

I assume customized kernel as well? and where does the overlay drivers come from? How much custom back-ports and custom development?

The article is not on point to say GKE replaced docker entirely then... but you are not point to deny and pretend that you are running Docker on anything remotely common.