So, Flatcar is to CoreOS as Centos is to RHEL (if you ignore that CoreOS is going away). The differences make it not entirely analogous, but it's probably an easy way express it in very few words.
But, yes, in the beginning we simply removed the CoreOS trademark similar to how CentOS removes the RHEL trademark. But very different from CentOS, we knew from the start that the upstream would eventually go away and all maintenance would be carried by Kinvolk and other contributors.