Docker wants to grow (a direction that's out of their hands, as they have a board to answer to). They must do this while playing the delicate balancing act of tightly controlling the project's direction towards their desired list, while at the same time maintaining the goodwill of its community. Part of this is defensive enterprisey blog posts, part of it is singing their praises at their conferences, and part of it is reigning in fractures in their platform back under their own umbrella with a plugin model. I'm not even judging Docker for doing these things; as a funded company they almost have to do these things.
CoreOS wants to ride the same wave, but they know that Docker is encroaching on their turf and will increasingly do so (see above). That's why they're deliberately breaking away with their own format. Core hopes that they can lock away enough of an audience behind their platform before Docker can do the same. They basically had to do this to survive, because Core, being composed of modular components, can't make money when everyone takes their moudlar components and simply runs them under the umbrella of Docker's growing platforms.
And this whole API wrapping thing? It's a power play. Whoever controls the UI holds a captive audience -- literally -- whether that's Docker, Core, or someone else. This is basically war over your servers, though I'm sure nobody involved will commit the PR suicide of admitting it. You are encouraged to engage in healthy skepticism and wonder if this comment itself is written by one of the players.
I have no idea who (if anyone) is in the right here, but I encourage everyone reliant on these platforms to see what's going on beneath the waves. Be mindful of the incentives these companies have, and make your technology choices accordingly.