Yeah, that's emerging as the most likely explanation. Six months ago, hardly anyone - including in NK - had even heard the younger Kim's name. Next thing you know, he's a 4-star general despite having no military background. Then, a couple of weeks ago, NK published a photo showing KJU sitting immediately at KJI's left side with senior military figures standing behind, a tacit message that he was second in command. I think just in the last week or so an official in the North publically mentioned KJU's name in context with succession.
But KJU is still unknown even to NKoreans, so, yeah, he needs to gain some street cred somehow. This is the NK 'legend-building' method, seems.
The big question is whether or not the real military will accept KJU, a 28-y.o. with no experience, as their Commander-in-Chief, so to speak. In Confucian countries, age and tenure determine rank as much or more than actual accomplishment (of which KJU has none, anyway).