Yes, if they drop a one-megaton H-Bomb, it may well be considered alarming. That's the second half of the message of this film produced by the British government for public consumption. The first half offers the message that it's just like being hit by a fission bomb, only more so.
Nothing to see here. Move along. It was a standard message of PSAs about nuclear warfare, offered in soothing tones that also pointed out that people were working on plans to deal with the aftermath. Was this supposed to be comforting?
I think it was a continuation of the sort of films produced by the government during the Second World War, but without the defiant optimism. Which leave you with that empty, sinking theory, a vague hope that it doesn't happen.