Unironically though, most socialists have historically been and still are for equality (especially ones who haven’t read Marx and are in it for the vibes), so this is a perfectly valid criticism and one Marx himself made towards Lasallean Socialists in Critique of the Gotha Programme:

But one man is superior to another physically, or mentally, and supplies more labor in the same time, or can labor for a longer time; and labor, to serve as a measure, must be defined by its duration or intensity, otherwise it ceases to be a standard of measurement. This equal right is an unequal right for unequal labor. It recognizes no class differences, because everyone is only a worker like everyone else; but it tacitly recognizes unequal individual endowment, and thus productive capacity, as a natural privilege. It is, therefore, a right of inequality, in its content, like every right. Right, by its very nature, can consist only in the application of an equal standard; but unequal individuals (and they would not be different individuals if they were not unequal) are measurable only by an equal standard insofar as they are brought under an equal point of view, are taken from one definite side only – for instance, in the present case, are regarded only as workers and nothing more is seen in them, everything else being ignored. Further, one worker is married, another is not; one has more children than another, and so on and so forth. Thus, with an equal performance of labor, and hence an equal in the social consumption fund, one will in fact receive more than another, one will be richer than another, and so on. To avoid all these defects, right, instead of being equal, would have to be unequal.