The taxation of land rent is a tax in form, but not in substance. Consider what happens if government builds a subway. Land values rise downtown because there will be more commerce, and land values rise in the suburbs because it is now much easier to commute to downtown. So people are paying higher rent or mortgage payments in order to be located where the subway is. But they also paid for the subway in the first place with taxes, since it was financed mostly from taxes that come from their wages.
Folks pay twice for the subway, first through taxes, and then through rent.
If the tax is the rent, then people only have to pay once. Therefore, the single tax is single in being a single payment for government services, rather than the double payment that takes place when wages and profits are taxed. A tax only on rent is a twice-single tax, single in terms of there being only one type of tax and single in being only one payment for government services. Aside from removing a burden to growth and enterprise, there is another benefit from the rent tax in being single. With only one tax, people can tell just how much they are paying for government.