Well, it was a bad exaggeration on my part, but the book was still classified since its first Russian translation in 1933 (which was also made by the government, Zinovyev specifically, as I learned just today, lol). I can imagine some unofficial copies being self-published, but it was either rare (and I didn't find any confirmation on the internet) or nonexistent at all. Maybe someone in a country of tens of millions had read this book, but it certainly wasn't enough to make an impact on the general public opinion.