It is far-fetched 'cause it's not AFAIK true. My experience for this is having set up a GKE Kubernetes cluster last night, it was definitely running Docker.
From one of the GKE folks: https://news.ycombinator.com/item?id=13715358
It's not true.
Google running their own containerization tech with a Docker interface seems a bit far-fetched given the level of integration of Kubernetes with Docker. That's totally possible though, I'd like to read more about it.