I think the title should be "Kubernetes is going to support X" where X is either rkt or appc. CoreOS is a company, developing an open container spec (appc). rkt is CoreOS's implementation of that spec, but other implementations are possible. Currently X=CoreOS, which is most likely to imply CoreOS's operating system, and that isn't what the article is about.
We've updated the title. We can update it again if there's a better one.