bluffing a threat to the plane by snapchatting your friend would be a weird move. No one on the plane even knew a "threat" was made.
It seems like "we have no details at all about the threat (because it wasn't actually credible), so let's just be prepared for every situation" is the logic.
Guessing it gives them information on what is actually happening on the plane. If the pilots were being held against their will they could be following orders from a hijacker.