Vulnerabilities will always sell for more on the black market because there’s an added cost for asking people to do immoral and likely illegal things. Comparing the two is meaningless.
To give a straightforward answer: no, I don’t think $20k is underpaid. The severity of a bug isn't based on how it could theoretically affect people but on how it actually does. There's no evidence this is even in the wild, and based on the description, it seems complicated to exploit for attacks.
Although, maybe there is something to the immorality/illegality tax in this case. The author is in high school (how cool is that!?) and the article would probably hit differently to perspective employers if they were detailing the exploit they had sold to NK (which is to say nothing of how NK would feel about the sunlight).