"Marx certainly describes the system of robbery that is state debt as primitive accumulation."
Does he? He does credit the state with a key role in primitive accumulation, as the institution of coercion, and indeed does indicate that state debt and taxation are a burden on the worker. However, he checks himself (and others) with the following:
"The great part that the public debt and the fiscal system corresponding with it, has played in the capitalisation of wealth and the expropriation of the masses, has led many writers, like Cobbett, Doubleday and others, to seek in this, incorrectly, the fundamental cause of the misery of the modern peoples." Capital Vol 1, L&W, p 708
Elsewhere, Marx makes it clear that taxation will always be peripheral to the fundamentally unequal (if superficially equal) exchange between capital and labour, and that taxes are typically a tax on surplus value already created in production:
"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).