If we were sure of that, then we should call their bluff and expect them to surrender or their army fall apart..
The N. Koreans have made these political calculations many times and determined (correctly) that the western world is not going to wage war against them. As I wrote, the sabre rattling actually helps the leadership maintain control over the proles.
This is a winning strategy for the leadership. They have access to black market goods. They don’t suffer the same deprivation as the proles, and the constant state of near war justifies all manner of oppression. The proles, who are heavily propagandize from birth, won’t know the truth.
I’m just guessing, but I think the torpedo attack was meant to shore up the leadership. It’s unusually strident (in recent history), but similar in nature to shooting missiles over Japan. Perhaps it indicates a weakened government?