If you follow the strict definition, then they are socialist. They are not communist, because
1. They have a state
2. They don't have a classless society. They have the ruling class and the impoverished working class.
However, they are the most socialist state in the world, hence also the state closest to communism.
Some socialist/communists call them state capitalism, but that is just because they are embarrased they exist. North Korea is not a capitalistic society.