No you haven't. That is not what is written in the PATENTS file. The PATENTS gives you a patent grant to any react patent facebook may or may not have, that you can only lose if you sue facebook for patent infringement. Without the patents file, you don't have any grant to those patents.
> You have to immediately stop using React.
No you don't. Why do you think this is the case? There is nothing in the PATENTS file about this at all. All it says is that you lose the grant. If a court grants a preliminary injunction, then yes, you have to stop using it immediately, but guess what: assuming Facebook has some react patents, they can apply for a preliminary injunction against you whether or not the PATENTS file is in there. With just plain jane BSD they can also get a preliminary injunction.
> In addition, without an explicit patent grant, a patent grant is implied with the license.
It's totally misleading to state unsettled law as fact like this. It's wishful thinking. Not being a patent holder myself, I would like it to be true as much as anyone, but the fact is that until this stuff ends up in court, just assuming that a license to redistribute also implies a license to any patents is just a theory.
When Facebook explicitly includes a revocable patent grant (aka the PATENTS file), the argument of it being a "BSD license with no patent grant" when the patent grant is revoked is self-fulfilling. But if the PATENTS file had never existed, then what you have is a "BSD license with a debateably implicit patent grant", which has been the topic of debate long before React even existed, and has yet to be tried in court so could set a precedent, and is a way better bet for the consumer than a "BSD license with no patent grant because that patent grant has been explicitly granted and now revoked, no debate, period".
In situation B (React is licensed under just BSD), the patent rights necessary to use React are not written in explicit terms. You have made no explicit agreement to have your patent grant to use React revoked when you sue Facebook for patent infringement. In addition, without an explicit patent grant, a patent grant is implied with the license. You can at least continue to use React until the results of the court case.