I am pretty sure the “find your phone” service uses the cel network. Pretty sparse coverage up in the Hindu Kush.
True- once they got out of areas with coverage, this probably wouldn’t have worked. But coverage is sort of unpredictable.
I don’t recall exactly when it was first realized that the flight was missing, but I’m sure family started calling about that time. However, as you say, it may have been a waste of time.