Maybe, though my guess is that most people who use this phrase really don't know or even care precisely what they mean, beyond "cause a bigger catastrophy than

Maybe, though my guess is that most people who use this phrase really don't know or even care precisely what they mean, beyond "cause a bigger catastrophy than humanity has ever seen". It's the same for poeple talking about extinction from nuclear war or climate change. There is little support in "The Science" for any prediction of a fatality rate of 100% from either (given current arsenals in the case of Nukes). People simply do not want to imagine what the world would be after a large scale nuclear war. So if they're given advice that would actually significantly increase survival chances (such as duck&cover, having some store of fresh water, etc), they prefer to laugh it off instead of taking it seriously.