I think it's extremely unlikely that this kid got some fighter jets scrambled because he posted the joke on airport wifi and it was intercepted by advanced secret technology. More likely it was reported by a civilian, and the person so reported to panicked that they didn't want to be the one responsible for an attack they were warned about, considering the warning would surely be made public if the attack occured. If it was secretly intercepted, then it could have been dealt with more reasonably, which is to say, not at all.