I'm sure the fact that flow is considered more tired to react while typescript is considered more agnostic helps as well (I've not explored flow so I don't know whether it is tied to react or not but I do think that's the impression).
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/
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.