The phrase “organize themselves as a class” with respect to the “proletariat” is purely militaristic, particularly with respect to calling for a revolution to “make themselves the ruling class”, which is a coup d’état. There is no such thing as an “organic” alternative, whatever that is; please don’t attempt to insert neologisms.
If the left is somehow “post-Marxist”, it can only be due to Marxism per se being an anachronism.
Again, as previously shown, Marx and Engels had made clear that they rejected the idea of a secret organization that would launch a coup d'état against the State as the Vanguard. You're saying that this has to be, that Lenin was somehow not in defiance to Marx as this is the only logical sense for a Revolution to occur. The answer would be correct if Marx was concerned with thinking logically at a tactical level. Marx however, wasn't. He believed that Capitalism would inevitably be destroyed as it was simply the course of history. This magical thinking is exactly why Marx has been compared to Christian chiliasts who spoke of the immanence of the Second Coming. Marx was a true believer and in other circumstances could have easily been a cult leader or a religious fanatic.
If the left is somehow “post-Marxist”, it can only be due to Marxism per se being an anachronism
It is an anachronism. They have moved beyond Marx and Communism decades ago. Their thinking has evolved and their rhetoric and conceptualization has changed wildly.