You can easily combine techniques. Send rescue dogs and handlers wherever phone signals are traced.
Having worked for companies trying to detect mobile phones this would be about as useful as guessing. Triangulating a signal (if you could even get one from under 2 tons of rubble) is not an easy task in the real world, especially one that would need to be accurate to within 1m or so.
That's probably what they are doing. These people are not dummies that need to be lectured by some hacker news guys.
It would be a waste of resources to rescue an ownerless phone.
Which is why you'd correlate with lists of known missing and known not-missing people before digging in.
Missing people won't lose their phone in a disaster?
It is a waste of resources for rescuers to sit doing nothing because they ignore leads.
False dichotomy. You can do other things besides chasing a very high false positive-rate indicator and sitting still doing nothing.
Use your imagination. No one is suggesting rescuers dig for ownerless phones. They would be just another signal used to determine where trapped people might be.