Microsoft has been open sourcing a bunch of their programs for a while now too. Majority are inconsequential but they are still nice to see. People on Linux OS's are excited about Microsoft calculator being open source but these open source projects still show that some people there have interest in the push.
EGreg didn't mean to say that VS Code used to be Atom, or is based on Atom, though I agree his wording was a bit ambiguous and it could be interpreted that way.
During that time, Atom was released (2014). But I don't recall it ever being especially popular - at least outside of the JS ecosystem. For one thing, it was kind of slow on release (people still complain about Electron!) and while it offered a lot of customization, these customization often seemed to worsen its performance. It was VS Code that really seemed to draw a wider audience from my perspective.
That said, I switched to vim around the time Atom came out, so I may be out of touch. I doubt there are any solid stats anywhere...
Pick one.
Visual Studio Code seems to be their big open source push, besides GitHub. Everyone uses it, and most development environments and UX are based on it. Used to be Atom, I remember.