I think the "hoax" is itself partly a hoax. It's one of those exaggerated tales, like the Orson Welles War of the Worlds radio play. Supposedly there were thousands of credulous people writing in to inquire more about the spaghetti trees; probably a great many were fully in on the joke and were just dryly "yes-anding", as we say nowadays. Perhaps on behalf of their children, rather like how one would "write a letter" to Father Christmas.

50s Britain wasn't that ignorant of the outside world (especially compared to other 50s countries). There were hundreds of thousands of servicemen who had been in Italy in the war and for the occupation, and the Empire was mostly still around. People knew what spaghetti was.