Chromium OS is open source: https://www.chromium.org/chromium-os. OverlayFS is in the upstream Linux kernel.
CoreOS is a very similar idea, and given that, we don't make builds of the container images available outside GCE.
Customers who need specifics of the OS (that they can't find them by just looking at the kernel config on the node) are welcome to ping us a support ticket.
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.