Income inequality as an argument against the market economy is typically based on some assumption of the morality of egalitarian outcomes and not the zero-sum fallacy.
Some Marxists, for example, claim that capitalists are able to pay workers subsistence wages and extract their “surplus labor” for themselves. They don’t deny, necessarily, that workers do not gain subjectively from labor contracts they enter into. But, capitalists prevent them from sharing in the productivity gains that their labor creates in a growing economy.
Take a look at Murray Rothbard’s book: