Earlier quoted context omitted.
Free trade requires both sides to be equally free. Tariffs are one of the defenses against another nation massively subsidizing an export. Getting all countries to play fair (in this case neither subsidizing or taxing) isn't going to happen. You also run into the interesting situation of what is a subsidy that impacts exports/imports? For example, post great depression america started tax subsidy for farmers to ensur…
Hong Kong practices unilateral free trade, and it sure hasn't seemed to hurt them.
That is because Hong Kong makes a living primarily as a place where trades are executed, not as a significant trader itself. Hong Kong does not have large supplies of natural resources, huge manufacturing industries, or millions of acres producing agricultural exports. Pretty much the only economy they have is hosting relationships and taking a cut, so they have no economic reason to erect trade barriers.