The title of this article is as misleading as it gets (and for that reason, I've flagged this HN post). While CoreOS is developing a competing solution, they cannot just "break with Docker" since that is what most of their customers probably use now. In fact, CoreOS announced support for Docker 1.3.2 2 days ago, despite them launching Rocket:
https://coreos.com/blog/docker-1-3-2-stable-channel/
I think Rocket is just one attempt not to become useless now that Docker's scope is expanding. There was an interesting post about the predictability of CoreOS's move a few days ago on HN: