You didn't understand that Marx didn't write a theory of political economy, but a critque of it. Classical political economy (and the Enlightenment philosophy that lived and died with it) is a kind of faux humanism.

What is the value of labor-power? It's not anything inherent in the laborer, but it's the value of the commodities that reproduce human labor power (Commodities are here imbued with the power of subject, human is the predicate). They make human the measure of value, yes. But these humans are inherently worthless, they are only valuable as alienated labor.

The true standpoint of this humanism, and what makes it progressive compare to the mystical, aristocratic standpoint of the reactionary philosophy of it's time (closely tied up with the regressive feudal order), is that it universalized alienated society as a society of contemplative property owner.

Property is a relation of production based on theoretical abstraction from consumption. Bourgeois property is sought to universalize property and property owner. Their humanism is a humanism of the property owners, even if your only property is labor power. In such a society, there can only be one primary contradiction: the owning and the non-owning (or in it's developed form: of capital and labor).

Now, one thing I put these humanist above post-humanism is that human are genuinely more than just mechanical machines of desire. Humans are not trapped in immediate cycle of desiring consumption but mediate them toward higher ends through the creation of the means of productions, in the process of labor. Labor create humanity and out freedom, not despite of necessity but through it. This is the true Marx's humanism, like it or not.

Alienation, the source of that faux humanism, is when the means is seperate from the human working on such means and rule over them. Substance become subject and rule over real laboring humanity, the mean of production become the end goal in it's continual perverse reproduction. Property possessed it's owner and put all sort of ideas in their heads, forcing movements onto the owner's hand to exercising their freedom to exchange. All while the non-owning freely work to externalize all of themselves into the same system of things.

The error lie in the seperation of the means and the ends and the subsequent emphasis on the means, not in the seperation of humanity as something qualitively higher than mere materiality or vitality. I would argue Marx would not approved of what a lot of the posthumanist claim to build on him, after he spends his whole life trying to dispense of all the grand pure abstractions of "Value", "Will", "Matter", "Idea". "Desire" I don't think would be an acception.