IANAL but I don't believe this is correct.
If I'm using React and I decide to sue Facebook for infringing some other software patents I hold, I lose the patent grant for any potential patents Facebook might hold concerning React. Which they could go after me with if they do indeed hold such patents. There's nothing proactively forcing me off React.
I just don't see anything but good from this BSD+patent grant situation. It lessens software patent litigation in general and lets me know I'm safe from Facebook suing me specially. It seems fantastic!