Yes, it's officially atheist because there's only room for one god figure, who happens to be a man. Christianity and Islam are "officially atheist" in the same absurd way. In NK the one permissible exception is not called Allah or Yahweh but Kim.
You know, the guy whose portrait hangs in everyone's home in the exact same spot where you'd find a crucifix in a southern American home.
And no, the religious nature of personality cults is not a fallacy. If anything, No True Scotsman applies to claims that a personality cult is not a "real religion." They are absolutely indistinguishable from theistic religions, except for the minor, ignorable detail that the god is alive and walking around.
Of course there's also a strong component of ancestor worship in the cult of the Kims. The portrait or other object of veneration is as likely to feature il-Sung as one of the other two.
Go to North Korea and try to sell them atheism, for instance. They'll send your remains home in a cardboard box.