We now accept Stalin’s slaughter as obvious fact, but at the time the full scale of the horror wasn’t widely known. Conquest was a pioneering figure, then, in documenting the reality of Soviet rule. That is the source of the apocryphal story about him.
Here’s a common version of the story as recounted by the Guardian:
When his history of Stalin’s purges, The Great Terror, was republished after the fall of communism, his American publisher asked him to suggest a new title. He came up with “I told you so, you fucking fools”.
Sadly, though, it never happened. According to Conquest himself, his friend Kingsley Amis (the famous British writer) made it up.
Not quite that Amis made that story up, but that Amis quipped that that should have been the title. And he was right, too.