This examples fails to account for the fact that radiant floor heating is a default feature in all new Korean homes.
The article is about Japan. I would expect Korea has a different climate which means different considerations. (but I'm not an expert in climate)
I'm not sure that's true. Hot air can hold a lot of moisture, the extreme example being steam.
Most comfortable place I ever lived in, heating wise, was an old house that had been converted into apartments. It still had steam heat. I loved that.
Steam heat is radiant which feels very good. Nothing to do with moisture added, steam heat is almost always a closed loop so no water is put into the air.
Hot air can hold a lot more moisture than cold air. (what you said). However when you take cold air and heat it the amount of water in the air doesn't change, while the amount of water than the air can hold did. When talking about water in the air the important part is how much it can hold relative to how much it does hold, not how much it actually is holding.