Weird that they would assign it a value of 2% if it's "no data"

If these conclusions are backed by science, then they are based on the collection of DNA. A scientist would need to sequence the genomes of thousands-maybe tens of thousands-of random people from all across the country. The more populous the country, the more random samples would need to be collected to derive meaningful results.

...this imposes limitations on which countries could be analyzed. Smaller countries would be easier, and they'd need to be of some economic or scientific interest to justify the cost. In a country like North Korea, only the government would be allowed to collect this data. If they were willing to spend the money, I doubt they would release even anonymized results.

North Korea is coloured in, looks like the grapher lumped them in with South Korea…

Why do you think it was some pure ethnostate in the first place, and why wouldn't Chinese people count?

There might simply be another ethnicity than "Korean". Perhaps "Manchurian".