I walked into an NK submarine from 20 years ago. I was not impressed. It had a regular old VCR sitting on a regular shelf like in a motorhome. And construction that would make a Victorian pipefitter blush. It ran aground during routine operations which is not so surprising given the utter lack of precision instruments evident. I would be surprised if in the past 20 years they went from that to a usable nuclear missile platform.
Your link explicitly mentions that the equipment found in this specific vessel is probably not the original one:
Speaking of equipment, it was obvious the submarine was stripped of any useful electronic equipment, such as navigation or communication equipment. In its place were items taken from a dump.
I imagine that all the original equipment was removed and analyzed by a South Korean Intelligence Service before the Sub became accessible to the public.
And ballistic missile subs of the world: http://www.hisutton.com/Nuclear%20Missile%20Submarines.html