I don't agree with this, I have researched English proficiency across the world and South Korea consistently ranks quite a bit higher than Japan. Here is one example:
https://en.wikipedia.org/wiki/EF_English_Proficiency_Index
I also regularly talk with many people from both Japan and South Korea (being that I learn the languages), with those people having wildly varying degrees of English literacy... but in talking to all of them, the one consistent thing everyone has agreed on is that Koreans by-and-large definitely speak better English than Japanese.