In this particular case, I think the point is less 1 or 2 but more point 3
(3) the contrapositive, where you continued the flight, it really was someone stupid enough to name the broadcast name of a bomb "BOMB", it goes off, and now you have to explain to the press "we thought nobody would be stupid enough to really name it 'BOMB'"
So you assume it's a low risk event, and tell everyone onboard to turn off their devices to remove the chance it's just someone making a bad joke or a coincidence, and then you end up with the outcome of trying to avoid having to say that in a press conference where everyone is already primed to think you didn't do enough.
(3) the contrapositive, where you continued the flight, it really was someone stupid enough to name the broadcast name of a bomb "BOMB", it goes off, and now you have to explain to the press "we thought nobody would be stupid enough to really name it 'BOMB'"
So you assume it's a low risk event, and tell everyone onboard to turn off their devices to remove the chance it's just someone making a bad joke or a coincidence, and then you end up with the outcome of trying to avoid having to say that in a press conference where everyone is already primed to think you didn't do enough.