The bit in questions is right at the beginning (and immediately followed by "Now, all kidding aside.." which does lend credence to the claim that it's a joke).
The source of this information is the website 38 North, which has been doing detailed reporting on North Korea since 2009. Here is their report, with a lot more details and photographs: https://www.38north.org/2018/06/yongbyon062618/