Any kook or quack can claim that their ideas are "scientific", but that does not make them so.
Very often, kooks use a language similar to that of science and claim a scientific basis for whatever they are advocating, but their purpose is to lend credibility to their ideas so as to impress people who are naive about the methodologies of real science and do not understand its language.
In making a judgment about the scientific validity of such a claim (i.e. that socialism is based in science), one must look at whether there is supporting evidence. True, I have no desire to study Marx, but I have never heard any claim that he undertook any kind of hypothesis-driven scientific study (whether observational or experimental), the carefully analyzed results of which led him to the conclusion that socialism is, in fact, a natural and workable model for human society.
A statement out of your quote, "For these misty formations in the brains of people are necessary sublimations of their material, empirically ascertained life-process, which is bound up with material conditions," is as good an example of pseudoscience as I have ever seen. He put sciency words together, but they express no coherent science-based principle or observation.
Popper's falsification philosophy was largely in reaction to both Marx' and Freud's claims that their ideas were scientific - by extension their followers would claim equal standing to the theories of physicists, and in particular to Einstein's.
Their claim was based largely on their "explanatory power" which was due to the ambiguity of their theories and how stories could be written or rephrased to dismiss any challenges. Popper's science as falsification showed that the true value of a theory increases as the theory survives repeated attempts to falsify it.
Popper did not apply his point to the historical sciences, but he could have.
"Explanatory power" will not do - make a "just so" story vague enough and it can explain away most any challenge but it does not make the story science.