Doesn’t need to. Being in a Faraday box is a reasonable trigger for a single reboot. That said, the most incredulous part of this story is that iPhones can detect when they’re in a Faraday cage.
I'm even more confident that Apple hasn't spent the research hours required to do that reliably, then incorporate the electronics and software needed into off-the-shelf phones, all to protect criminals from having their phones hacked under very specific conditions. That seems like a huge money sink.
In a zero-signal environment? With other iPhones in very close proximity?
You can even measure your false positive rate by timing to first successful unlock. If it happens more than once, turn down the sensitivity on the feature (or turn it off completely).
(Were I designing this feature, I’d let phones in this state poll the other phones on how long they’ve been in it.)
Lack of motion? The information the other phones provide are proximity (it’s unusual for people to pile their phones together), that the radios still work and possibly a timeline, e.g. if the other phone says “I’ve been in a suspicious state for two days,” the first phone can change its priors.