Atom Shell is a runtime environment that you can write your own Node-in-a-web-browser apps against, yeah. Atom-shell is the C++y bits to provide some of the APIs that are available in JS.
Its got a _long_ ways to go before sublime has anything to worry about. The plugins and language support Sublime has is its real asset. I'd be happy to make the switch when the feature parity is there, because I'm not too happy with the way Sublime 3 has played out, but that day is a ways off.
Just really poor communication in regards to status, it blew past the 2013 release window without any status update from the developer. The product is great, just feels poorly managed. It leaves me with an unsure feeling in regards to the future of the product.
The Atom Shell open sourcing is also interesting ... I wonder if it will lead to a rash of other Chromium-fork-apps