Especially when the experts spotted old Eastern Germany Technology on their new Ballistic missiles..... of course the subs will sound like a motorbike without an exhaust when it comes to tracking.NK has had a number of *experimental* missile submarines under construction since 1994 -- the Gorae "Whale" class. They are likely to have some basic operational capability by now, even if a long way behind Western standards.
They have also put a number of former Soviet "Golf" class ballistic missile submarines back into service which were originally bought for scrap from Russia, but the NKs have refurbished them and put them back into limited services.
Do not underestimate the Fat Boy.
If you have a spare moment.... google earth North Korea, zoom in and just check how little vehicle traffic is on their tracks/roads.