Perhaps Marx should have been speaking for himself, since the only individual nature he could really know was himself.
Human nature is too complex to be that predictable, especially where he refers to individuals. It HAS been proven that group behavior is predictable.
I read an interesting but sad report from Communist Russia. It said that an experiment - an evil, immoral experiment, by the way - was conducted on a group of children. From the time they were born, they were taught to live the collective lifestyle. The result - a classroom full of kids that did nothing but rock back and forth in perfect unison all day long.
The Commies got what they wanted: perfect human beings or perfect little robots; either way, they were perfectly adapted to the material conditions of their upbringing. Not that it's what they literally wanted, but they got out exactly what they put into the experiment, only they couldn't predict it until it happened.
Communists don't understand human nature. Marx didn't understand human nature. They think they do. Thus they conducted immoral experiments on little human lives, and a generation of children was destroyed.
Marx, unlike Rand, focused on alienation (this was a tactic favorable to Existentialists). Neither Marx nor Rand were in favor of alienation, but both of their philosophies produced it nonetheless. Marx focused on science, but he had no science or experimental evidence to prove that capitalist economics produced alienated individuals. His focus on science was therefore a sham. But when experiments are introduced, somehow the result is always failure. And there's never any reason to judge Marxism itself a failure, they simply try again, and again, and again, producing more and more failures, destroying more and more generations of human beings in the process.
If alienation of individuals is bad, then how does focusing on collectives reduce alienation? How does forcing millions of peasant farmers into communal farms reduce alienation? How does taking away a people's independence reduce alienation? Marxism cannot answer these questions. It dogmatically asserts that it can happen, because human nature is infinitely malleable to its purposes. But what if human nature is not infinitely malleable?