That's quite inaccurate. It's is like saying that OpenStack started out as an extension to KVM. Both had very different (original) scopes.
> Whether the Kubernetes community continues to build primarily on Docker, or switches to containerd
Kubernetes has 3 container runtimes at the moment: Docker, Rocket, and the new remote runtime which allows you to plug in a separate server using a protobuf interface. There is a lot of focus on the remote runtime now. The CRI-O project for example is implementing a remote runtime that allows any OCI conformant runtime to be plugged into Kubernetes.