Why it is remarkable that Korea and Taiwan surpassed Japan

To be fair, Korea and Taiwan are not the only cases where former colonies have become wealthier than their colonizers.
Countries like the US, Canada, and Australia are settler colonies established by migrants,
while Hong Kong, Macau, and Singapore are very small city-states.
However, strictly speaking, Korea and Taiwan were resource-exploiting colonies.
So, if you look at it closely, Korea and Taiwan are the only ones that have surpassed their colonizers from a status of being a colonized nation. Of course, due to the limitations of their sheer scale, they haven't surpassed the total economic pie itself.