It's obvious Today that it is not true but in 1969 (I was 12 then) it seemed very plausible, at least to a 12 year old.  I think It was a hoax perpetrated by Lennon at the prompting of Yoko maybe.  Lennon had an exquisitely ironic, almost sadistic sense of humor.  Also, the Sixties was a very strange and dark period.  Mass media, i.e. television, was reletively new.  People were always launching hysterical rumors and usually found enough niave listeners to start a panic.  After JFK was assasinated there were rumors almost immediately that it was a pre-emptive strike by the russians who were about to launch world war III (this was shortly after the cuban missle crises, which was also rumored to have nearly started WWIII).