I suppose the advice is assuming you weren't in the range to be killed by the blast at the start, only within range to suffer from the fallout if you don't shelter. But yes, it's a pretty scary prospect. I expect I'd be affected if one was dropped on London, but maybe not by the initial blast, only by fallout, because we're 45 minutes by train out of the city. If they dropped one on Air Command though, we might never live to tell the tale.