From the article:

    [PotUS] letter to South Korea, alleges that the country’s "Tariff,
    and Non Tariff, Policies and Trade Barriers" are responsible for the
    bilateral trade imbalance:

    South Korea has had a free trade agreement with the United States
    since 2012, so most U.S. exports to Korea face zero tariffs. 
    Japan, like other wealthy nations, has very low tariffs on most goods.

    Neither country [has been] in a position to offer big tariff reductions,
    because their tariffs were already minimal.

These seem like important details to me.