"We grant you a right to use all patents we have covering this work.

[1] If you sue us regarding a patent you claim covers this work, we revoke your right to use all patents we hold covering this work.

[2] If we sue you regarding an unrelated patent, you retain your right to use patents covering this work for purposes of this work.

[3] If you sue us regarding an unrelated patent, you retain your right to use patents covering this work for purposes of this work."

The entire point of people being annoyed about this is that Facebook specifically didn't include the [3] grant. Consequently, this was more about Facebook attempting to use React's popularity to make themselves immune to patent litigation (as anyone relying on React wouldn't have been able to sue Facebook for any patent without losing the React patents grants).