There are many areas in the article where they describe their plan to open source this. It's in their roadmap for v0.4 projected to be released in Oct. 2014.

Also, "...This is the reason why we decided that once the core framework is implemented we will release the framework code under the Apache License Version 2. We already have an organization on http://github.com/NativeScript where the project will be hosted and will be pushing the code shortly - now just go ahead and star us."

... from the section titled, "Open Source - not yet another proprietary framework".