If you sued Facebook, you would have lost the patent grant, but not the copyright. With the new license, there's no patent grant at all, so if any code in React is patented, you're in the same boat as you'd have been if you'd sued in the prior situation.
Edit: It seems like some consider MIT, unlike BSD, to have an implied patent grant based on its wording: https://www.scribd.com/document/46088081/Closing-the-Loophol...