Only difference is now the government says "give me your stuff or I beat you" rather than your neighbor
Oh, since we can't have them benefiting from positive outcomes of collective action, we'll pipe in polluted air and river waste to help simulate a truly free market environmental policy.
That’s a completely artificial and shoehorned idea. If you want to prove a point about libertarians, do it properly in a closed system. They must be left to their own devices and are left to clean their own pollution.
That's the point libertarians don't get: there is no closed system. There is no green field hypothetical.