The fact that you still have to apply as a developer is a bit discouraging. (though that might change...). I can't seem to find anything on github.

Hoping that this is an oversight and that they will open-source the core platform in the next few months/years. It would be so awesome to hack/build on this platform

It says 'to more device makers', not all, and anyway it wouldn't even have to be open source but just public releases? I expect it's a case of applying and meeting whatever standards though.