Answer: An export subsidy achieves cheaper goods in foreign markets while an import tariff makes imported goods more expensive.
Explanation: An export subsidy and an import tariff have different goals. For the domestic market, subsidies reduce the price of domestic goods and tariffs increase the price of foreign goods, making domestic goods more competitive, more desirable and fueling the domestic market.
When an imported good has enjoyed a substantial production subsidy, the price will be so low that tariffs will not prevent it from being sufficiently competitive in the foreign market against their domestic goods. In this case it achieves the same goal as a production subsidy.
Both ideas seem contradictory at first, but are both true.