Fair point. I wanted to clarify the difference since "built on Atom" has a different meaning to me than "using the GitHub Electron shell". The first sounds like an extension to Atom and the second sounds like a whole new app.
They started work on this before the Atom Shell was "Electron" and speaking to the team in Zurich before this was unveiled (about 3 months ago) they mentioned this was built on top of Atoms foundations. There is an Atom binary inside the package app and this is where my comment came from (: