Very limited “use”; mostly just enabling external developers to interface using React to their existing tools (that are not using React). If the Armageddon should come, and Microsoft is to enter a legal patent dispute/battle, stopping this React “use” is a no-op for Microsoft.
Microsoft builds Skype on React Native. They use React in production on several websites including outlook.com. They are using it internally and not just for external devs.
You'd be actually surprised how many teams inside Microsoft wrote their own smaller versions of OS libraries. It happened a lot less after Nadella era, but Ballmer era had a "we can't use anything not built here"