Using CoreOS as a "base image" under docker would serve little purpose because CoreOS isn't really designed to run anything other than its core services. People usually use Ubuntu or CentOS.
He's saying that there isn't a point of running CoreOS as the base image inside of a container, which is true for most people. There isn't a good reason to run CoreOS containers on a CoreOS host.
Oh! I had a different sense of "base ... under Docker" in mind.
My interpretation was that Docker was running on top of an OS, which you could call the "base", and say it was "under" Docker in the sense of "underlying".
Whereas he was using the opposite spatial metaphor: the Docker image, which is a "base" in the Docker sense because you derive other images from it, is "under" Docker in that it's within Docker, or lower in the process tree.
George Lakoff would be thrilled by this example of conflicting metaphors.
