Political Memes

You haven't read the relevant things, like Stalin's short history of RCP(b) where he explains what he thought (by the way, that's the real starting point in literature of Maoist ideology, Castro regime and so on ; Marx and Engels those people didn't like, and even Lenin was too wordy), and such.

It's the other way around, Soviet ideology, Stalin included, till at least 70s was more thoroughly Marxist than anything else really implemented.

It's just that most of the western leftist groups don't like how it went, so they pretend USSR was something alien. With the notable exception of communists in France.

Also I don't think fair pluralism is part of Marxism in any way. It's similar to modern leftist perception of the USSR.