They use the underlying core, but the code is their own. One benefit I see is that Microsoft gets to provide bugfixes to Electron that Github may find useful. As well as Microsoft may just end up porting some of their products (Office) based on their experience learned by developing for multiple platforms.
It is a completely different editor/debugger, it just uses the same Chromium wrapper, Electron, which was developed by the Atom team for Atom.
It's based on Electron but it is not using Atom no