Also, you can countersue Facebook as long as you don't sue over patents you have on React. You also can't sue others over patents on React (without losing the grant).
Also more importantly, the conditions don't apply to the BSD license. The patent grant is a separate patent license on top of the copyright license. The license the patent grant refers to is the patent grant itself.
In other words: if you don't sue us, you don't need to care if we have a patent that covers some React mechanism. If you want to sue us, you should first check if we have any React-related patents we could use against you.