You might be right; I don't know. But are you sure? I knew some Koreans some years ago. The family had been in the US embassy of South Korean and decided to stay in the states and become citizens. The father was from North Korea and his family came south before the war. He told me how Presbyterian missionaries had spent a number of years in Korea during the latter half of the 19th century evangelizing there--it's the reason why so many Koreans are Presbyterians today. I wouldn't think that only occurred in the south.
I remember Corrie Ten Boom writing in one of her books how surprised she was to find believers wherever she went. She specifically mentioned a horrible prison in Africa and Cuba. I just don't assume that there are no Christians in a country where God's people have spent time. Not only that, I remember a Korean coming to my church when I was a child. He remembered looking into the sky and somehow knowing there was a Creator. So he went in search of someone who could tell him about that Creator. I just don't believe God's word returns void. I can't. But that doesn't mean that we necessarily see how He works.
There may be some Protestant tradition in both halves of Korea, but overwhelming Buddhism and Shintosim was the religion on both Koreas prior to WW2, though of course Christianity has taken off throughout South Korea now. But since the Kims enslaved NK in 1949, they have all but wiped out whatever vestiges of Christianity were left. I suppose some among the very aged will have some long ago tie to Christianity, but they would be a distinct minority and certainly wouldn't dare practice it or even mention there past history. They'd be put into Gulags if they did.
Check out the book "The Aquariums of Pyonyang." It's a stunning insider account of NK's political prison system. Many who professed faith were tossed into it and murdered.