That clause relates to the patents you own that you're suing Facebook over, not the patents Facebook owns that they're (no longer) granting you a license to.
As an example, say Facebook owns patent 1234567 "Method and system for reacting natively". Google owns 7654321 "High-density computers in a data center". Google builds the new Maps app using React Native. This would infringe 1234567, except that React Native comes with a patent grant, so the Maps app is properly licensed.
Then Google sues Facebook over Open Compute Project, claiming infringement of patent 7654321. This has nothing to do with React Native, but Google has just lost its license to 1234567. So Facebook now gets to sue Google over the Maps app. (More likely, Facebook will threaten to sue in order to force a settlement for the other lawsuit.)
As an example, say Facebook owns patent 1234567 "Method and system for reacting natively". Google owns 7654321 "High-density computers in a data center". Google builds the new Maps app using React Native. This would infringe 1234567, except that React Native comes with a patent grant, so the Maps app is properly licensed.
Then Google sues Facebook over Open Compute Project, claiming infringement of patent 7654321. This has nothing to do with React Native, but Google has just lost its license to 1234567. So Facebook now gets to sue Google over the Maps app. (More likely, Facebook will threaten to sue in order to force a settlement for the other lawsuit.)