It does have built in type definitions for React, where as all other libraries you probably need to use flow-typed (similar to definitely-typed in TypeScript).

So it does kind of have React integration, but it's pretty loose - you can use it with any other framework, and there's nothing obviously React-specific about the language.

Edit: See https://flow.org/en/docs/react/types/

It's very tied to the react / facebook ecosystem.

Nuclide for atom was the official release by facebook for supporting flow + react, and now you have Atom-IDE which is optimized for react + immutable.js + flow.

Plus the facebook guys are avoiding VScode like the plague and focusing entirely on Atom. Which now also comes with the BSD+ licence.