"Anwar Shaikh would counter that taxes on capital are ultimately paid by labor. I don't buy it, but that's what he says."
I think Marx said sort of the opposite
"If all taxes which bear on the working class were abolished root and branch, the necessary consequence would be the reduction of wages by the whole amount of taxes which goes into them. Either the employers' profit would rise as a direct consequence by the same quantity, or else no more than an alteration in the form of tax-collecting would have taken place. Our argument is that although some taxes are paid by the working class, the burden of taxation rests on the capitalists and has to be paid out of the profit accruing to them in the form of rent, interest and profit, the basis of which is the unpaid labour." ("Criticism and Critical Morality", Marx and Engels Collected Works, Volume 6).
... that last phrase, though would justify Shaikh's word 'ultimately' in the sense that the source of the profit from which the capitalists pay tax is the workers' unpaid labour, then, yes, workers unpaid labour is the source of all government revenue.