The claim is based on a 1904 treaty. Notice this is just before Japan annexed Chosun by force. That is, Japan's claim is based on exploiting a time of weakeness. It is a case of sweeping generalization in the name of debunking a first generalization. Notice none of these points are mentioned in the article below.
Japan sweeps history under the carpet.
Japan sweeps history under the carpet. Japanese are sneaky and unfair as proved by their unfair business practices.