This came up in one of the articles:

Korea has a strong Catholic presence with a unique history. Unlike in many Asian countries, the Korean people became Christian prior to the arrival of any missionaries. They learned about the Bible on their own and “self-converted” before seeking instruction from missionaries in China.

For what it’s worth, other flavors of Catholicism and Christianity evolved outside of Rome’s oversight or with very limited contact. I’m thinking in particular of some of the east African sects, as well as Celtic outposts in Ireland and Scotland in the Dark Ages.