After they asked him in a serious tone to repeat what he’d said, he should have gotten a clue that they weren’t going to handle it as a joke. Most likely if he’d said at that point “I’m sorry, I was joking but I guess it’s not very funny; feel free to search my bag, there’s nothing unusual in there,” they’d have let it drop. Honestly, in a huge crowd situation, in a location which is clearly a likely target for Islamic terrorists, somebody saying anything about a “bomb” is apt to panic some nervous person into screaming “Oh my God! There’s a BOMB!”, and starting a dangerous stampede. I don’t blame Disney and the police for taking it seriously. A panicking crowd can be just as deadly as a real bomb. Try yelling “Fire!” in a crowded theater, and you’ll get hauled away by police just as fast, for the same reason.
We were not in the situation - apart from immigration officers and pub-law ther’s certainly stupid people trying to be funny on the list of things that are not funny.
(It’s actually a Bob Hope citation: Thinking of yourself as funny isn’t funny)