It's hard for them to claim this is "propaganda" when the same myths that this debunks literally weren't invented yet when the book was written, why would they make propaganda to debunk a claim that doesn't even exist yet? This isn't a slam dunk for everyone, more the sort of people willing and interested in reading the book, who are willing to examine their misconceptions and incorrect ideas about the Soviet Union, to a True Believer in anti-communism there's nothing you could say or do to change their mind, they wouldn't care about things like the "linear passage of time" disproving their claims.