Normally I would be the first person to cast skepticism on anything North Korea said.
But in this case, doesn't it seem pretty reasonable to say they have no cases? They had extremely limited travel from other countries before Covid starting spreading, and internally people do not travel that much within the country. Combine that with a real doctor level of control over people wearing protective equipment at all times, and I could easily see whatever small number of cases they would have had, would have been contained long ago and then there simply was no interaction with other countries to bring the virus back in.
Basically, they were already extremely isolated from the world before, and that has protected them in this instance.
They do/did have a decent amount of travel to China though, so that could quite likely be a quick transmission vector. With the overbuilding of Pyongyang and other cities crowding isn't really an issue there, but with the almost medieval level of civilization out in the rural villages if the virus took hold there things would not be good. And of course if the virus got into their reeducation camps it would be a decimation (in the literal sense).
Also not hard to figure out which people those 700 are that are getting tested weekly.