It would have been an idea to link to the actual article so others could read the passage within the greater context.
Here are the paragraphs that proceeded your quote and are part of the very same section.
How to Desocialize
Drastically Lower All Taxes
Another implication of our previous analysis is that taxation should be cut drastically. There is, in the literature on taxation, far too much discussion about which types of taxes are to be imposed, and who is to pay them and why, and not nearly enough on the height or amount of taxes to be levied. If the tax rate is low enough, then the form or principles of tax distribution really makes very little difference.
To put it starkly, if all tax rates are kept below one percent, then it really does not matter much economically whether the taxes are on incomes, sales, excises, property, or capital gains. It is important instead to focus on how much of the social product is to be siphoned off to the unproductive maw of government, and to keep that burden ultra-minimal.
While the form of taxation would not then matter economically, it would still matter politically. An income tax, for example, however low, would still maintain an oppressive system of secret police ready and willing to investigate everyone’s income and spending and hence his entire life. Economists’ opinion to the contrary, there is no tax or system of taxes that could be neutral to the market.[3]
Whatever taxation that might exist after desocialization should, however, be as close to neutral as possible. This would mean, in addition to very low rates and amounts, that the taxation be as unobtrusive and harmless as possible, and imitate the market as closely as it can. Such imitation might include the voluntary sale of goods and services at a price, or setting a price for participating in voting. The sale of goods or services by the government would, of course, be drastically limited in our desocialized system, because of the enormous scope of privatization of government activities. Privatization will be treated below.
That would be because he does not know the future.
Essentially where governments operate they crowd out or pass laws to beat / restrict the competition, that wouldn’t be the case for a failed state where it is being de-socialized (afterwards). It was a possible method suggested for making taxation as neutral as possible.
It would not be a monopoly. Consider the context. There is obviously the issue of it not remaining one, which we all know Rothbard understand - but that is outside the point of discussion.
But Rothbard was for voting. Ah! But here he is discussing a how to properly de-socialize a failed state i.e Soviet Union. As he mentions "that the taxation be as unobtrusive and harmless as possible, and imitate the market as closely as it can." Cue Misesian analogy of the market and voting.