I don't think you can directly compare Japan and Korea; the rain was much heavier in Japan and when it did stop the track drained fairly quickly. The Korean track was consistently wet; when the top drivers including Fernando Alonso, who had more to gain than the Red Bull's said that the track conditions were the worst he had driven in!
I don't recall heavy rain at any point in Korea during the pre race show. It seemed that the track was very wet and at most, a light drizzle. It started raining only just before the race got underway and even then it wasn't some heavy monsoon.
I've seen much worse in Malaysia, Brazil, in Fuji as well but the tracks handled it better. It was shocking to hear all the complaints from the drivers about this track in the wet and the "I have no grip I have no grip", I would atest that to the new track surface. In terms of rain volume, this was chicken feed compared to other track conditions in the past.