Argh! I did read about this at a bookstore the other day, in a collection of essays about the posterity of Mao’s Little Red Book. But no, I didn’t buy the book.
The way the book put it, the Communist Party in the ’50s was sympathetic to the aims of Esperanto, and saw it as a suitable, non-colonialist vehicle for getting their message out. I think the book subtly hinted that they were a bit naive about the propaganda efficacy of Esperanto. But in the ’50s and ‘60s, I suspect it was not that absurd a vehicle: most English-language vehicles would likely have been closed.
(Who was that American journalist who’d interviewed Mao in the ’30s, and Mao did an interview with to help prepare for Nixon’s visit? Not all English-language vehicles were closed; but the audience was certainly not as reflexively sceptical.)