The 18% figure is a biased sample from an anti-DPRK NGO. More comprehensive research into North Korean defectors by Cho Cheon-hyeon for his book Defectors indicate that most North Korean defectors simply want to make money in China, with only about 40% of defectors wanting to go to South Korea.
So I did misremember, but my point still stands on the fact that most of them don’t want to defect to South Korea, even before taking into account that even at their 2009 peak defectors were a tiny fraction of a percent of North Korea’s population and the existence of them in no way implicates all of North Korean society in secretly wanting to escape.