The Most Propagandized People On Earth

Same reason people keep referring to DPRK as North Korea, it's a result of decades of western propaganda. DPRK along with Cuba have been hands down the biggest targets of US propaganda because they remain as the closest analogs of USSR today. They utterly reject capitalism and don't even make an attempt to make any concessions to the west.

Is it a misleading translation of Korean honorifics or official titles

Thats what I've always assumed. Its a major tactic I've seen in reporting on China and the CPC, so i assume they'd use the same to the DPRK.

In the US politicians and officials (without a military rank) are formally refered to as "the honorable" so and so. Imagine how the US would spin that formality if it as used officially by a designated enemy. The average person in the US doesn't say that or would ever use it, but imagine the propagannda of saying Americans "their love their 'honorable leader.'"

Many (most?) countries have a prime minister. We're very used to hearing those words in conjunction, but if you just tell people it's a ridiculous phrase, then prime minister, supreme leader, dear leader, etc, are interchangeable in meaning and grandeur. Since you're translating from another language, it's up to you which variation to translate to.