Why does a tariff have no effect to an exporter? I'm reading Principles of Economics, the paragraph about tariffs in a imaginary country Isoland wrote: "The Isolandian economists next consider the effects of a tariff—a tax on imported goods. The economists quickly realize that a tariff on textiles will have no effect if Isoland becomes a textile exporter. If no one in Isoland is interested in importing textiles, a tax on textile imports is irrelevant. The tariff matters only if Isoland becomes a textile importer. Concentrating their attention on this case, the econo- mists compare welfare with and without the tariff."
I don't understand why a tariff have no effect to an exporter.
A distinction should be made between two different types of tariffs.

Let's define good imports as goods that enter into a country, across its border, for commercial purposes. Those imports could be subject to a tax on trade, called an import tariff. Thus, the Isoland country only taxes goods that enter. If no one imports the goods, this tax is irrelevant.

Let's define good exports as goods that move outward across a country's border for commercial purposes. Export tariffs would be a tax that a country imposes on its exports, which makes them more expensive. Isoland does not want to tax these goods because it would be a burden on its own businesses. 

Note, however, that these export tariffs can be put in place to encourage domestic consumption of domestically produced goods instead of exports. This could be the case in the context of a food or health crisis if a country wants to discourage exports. This is not the cas in your example.