It's the _default_, as they say in the article:

> There is no mandated development environment. Some developers use vim. Some use Emacs. And even more engineers use our internal, unified development environment called Nuclide.

Default is definitely different than mandated. Facebook provides engineers a ton of freedom for their development environment.

There are many at FB where we would have to pry vim or emacs out of their hands, and, even then, we would not be successful. :-)

Having a recommended/default IDE is not the same thing as forcing everyone to use it.

In every place I've been, the IDE choice has been more of a team decision, configuration was personal (some short-cuts, colors, etc). Too much time went into the build environment and other development tools for us, plus if you wanted someone to come in and help (short term pair programming), you need to be on the same page.