Well, the “why” is only known to that r/ultraleft user. Unless that user could share evidence of a policy detailing the ban, Marxist Literature is not banned in the DPRK as far as I know. Marxist literature is referenced in plenty of popular books of Kim Jong Il:

Maybe, the user is misinformed or misinterpreting the fact that the Juche idea(born from the teachings of Marxism-Leninism) is taught over other Marxist literature(twisted this information to reach the conclusion that it was “banned”). The answer that clears this confusion is simple and it is referenced in Prolewiki:

• “In the early years of revolutionary activities, the leader (Kim Il-Sung) was well versed in Marxism-Leninism. But he did not confine himself to applying Marxism-Leninism to the Korean Revolution but pioneered a new phase of revolutionary theory from a steadfast Juche-oriented standpoint and resolved the problems arising in the revolutionary practice from a unique angle.” -> https://en.prolewiki.org/wiki/Juche#Marxism–Leninism