Yes, your understanding is largely correct. To be clear, you can still sue Facebook for stealing your patented invention, but then your grant to any patents that your use of React infringes would expire and Facebook could countersue over that. Facebook legal's response to this concern has been largely obfuscation and "trust us" which is not encouraging. They did change the patent grant two years ago to allow you to assert patent claims against Facebook without losing the grant if Facebook sues you first, but that's of limited utility.
It's funny you mention Apple, because the Apple Public Source License has a very similar provision.
> 12.1 Termination. This License and the rights granted hereunder will terminate:
> ...
>(c) automatically without notice from Apple if You, at any time during the term of this License, commence an action for patent infringement against Apple; provided that Apple did not first commence an action for patent infringement against You in that instance.
The good news is that the Apple Public Source License is basically dead and not used for anything of importance.
Please explain to me - are they able to change it again at any time in the future to "unclear" it again or just change wording altogether? Will it affect all users retroactively in that case? Because all this "they had bad wording in license but they changed it so we can use it now" sounds kinda crazy.
The patent grant is solely a grant from Facebook to the world, so Facebook can choose to grant more rights at any time. Just like I can sign a contract with you allowing you to borrow my bicycle on Thursdays and then later unilaterally decide that you can borrow it on Mondays as well.
The University of California did something like this in 1999 when they cancelled the advertising clause of the original BSD license. See ftp://ftp.cs.berkeley.edu/pub/4bsd/README.Impt.License.Change.
It's funny you mention Apple, because the Apple Public Source License has a very similar provision.
> 12.1 Termination. This License and the rights granted hereunder will terminate: > ... >(c) automatically without notice from Apple if You, at any time during the term of this License, commence an action for patent infringement against Apple; provided that Apple did not first commence an action for patent infringement against You in that instance.
The good news is that the Apple Public Source License is basically dead and not used for anything of importance.