Being forced to buy protection doesn't mean you wouldn't buy it anyway; you just can't shop around.

Classic Libertarian argument. Government has a legal monopoly on the use of force that they employ to obtain tax revenue. The proof of the statement lies in what happens if you don't pay taxes. Eventually, someone with guns will place you in custody or garnish your wages or seize your property.

I am not claiming I haven't heard that argument before or that I don't understand it: I am claiming that I don't think this is what the person I was replying to meant when they said they pay taxes to help themselves and not other people, particularly given the explicit explanation "don't tax people in one country to help another".