This is sort of correct, but what you need to understand is by simply putting something on the web you're probably "violating" dozens of patents held by multiple big scary corporations. The big difference here is that FB has specifically described the patent grant, while most open-source projects leave it ambiguous.
Am I misunderstanding the flow?
1) Facebook has patents on some stuff React does. 2) (Assumption) Anything similar enough to be near-source-compatible probably runs a big risk of exposure to those same patents. 3) React is licensed in a way that you can use it without opening yourself up to patent litigation (it grants license to them). 4) Facebook infringes on some more-critical-to-your-business thing. 5) You sue. 6) Facebook sues you for using React (or Preact, from (2)?).
If you never use React you never get that patent license anyway, right? So be careful how distinct your replacement is. You'd need to analyze the claims of the patents you're worried about to see if they could still apply.
This guy seems to think the same ( https://medium.com/@dwalsh.sdlr/react-facebook-and-the-revok... ) but also doesn't feel like Facebook would ever use it, though that's not really relevant to the arguments made around the principal here.