Koh was not directly referring to Aang, but to the Avatar in general. In one of the Escape from the Spirit World mini-episodes, Avatar Kuruk, the previous Water Tribe Avatar, was introduced. He was a careless and free-spirited Avatar. This resulted in his wife getting stolen by Koh, as a punishment. Even since, Avatar Kuruk has ventured annually into Koh's lair in attempts to find his wife.