Assuming some NK hardware is not at least as capable is absurd.
This is 2016, not 1942. The technology for detecting and tracking submerged vessels has improved somewhat in the last 74 years. The North Korean subs are basically vintage 1960s era technology, like much of the rest of their military. They're not going anywhere without being tracked and if they approach the United States they will be sunk, war or no war, because nobody will be watching except the ones doing the shooting. Submarines can be accident prone and the Pacific Ocean has a fearsome reputation among mariners. Nobody would have any problem believing that a North Korean submarine had an "accident" while at sea. In fact, the visibility of submarines at sea to the general public is so low that they United States could essentially deny any knowledge. Finally, the North Koreans are so unsympathetic and unpopular these days that nobody would care what happened to their submarine anyway.
Well... here's some news from 2015