Ultimately duck and cover works for some lower yield nuclear bombs (or in the outlying region) and doesn't work in other instances. I agree that the mocking was generally misinformation.
Duck and cover works, just as walking fast, or walking up hill at all works in this tsunami scenario, it will save many lives, but of course it won’t save those that it doesn’t, meaning those doomed in the 100% kill zone.
Duck and cover is not to save someone from being vaporized but to save the many from what happens if they stand there exposed. Many Japanese would have survived, and many would have avoided blindness and terrible injuries, if they had been trained to duck and cover.