So basically if you have this awesome, let's say, middle out compression algorithm patent, and have an interface in react, then they can use your middle out without being sued? (Just trying to clarify what you are saying here..)
Your license to any patents which Faceboook may or may not have that cover elements of React will terminate. At that point, Facebook may choose to initiate a lawsuit against you over your infringement of whatever React patents (if any) they have.
So if you have (or are planning on obtaining) software patents, the React patent grant may not be very valuable. (Then again, Facebook may not have any patents actually covered by the grant, in which case it's not valuable to anyone.)
(1) Have patents / use a react alternative, such as vue (note this is annoying since there is no cross-platform native app version of vue ) (2) Don't have patents / protect your intellectual property in other ways (maybe by publishing a public whitepaper or something) / use React / Bust your ass since facebook can likely implement better and faster than any small team.