The over-simplification of Marxism and the history of Marxism is... Well, you can probably guess where I'm going with that, but I think there's something to his explanation, at least so far as it applies to Americans. But I think it's mistaken to regard differing reactions to reaction and Stalinism as a simple matter of proximity, as if people don't have some grasp of the broad historical distinctions between the two, as if they're motivated only by guilt and narcissism. It all plays a part, to be sure, but a greater horror towards Nazism than Stalinism is consistent enough across many societies with varied histories, to the point where a rejection of this distinction is widely understood as a clue that you probably shouldn't invite this individual out for a bagel, that we can't reject the actual, historical differences between the two, or the popular apprehension of those differences, in forming that reaction.