No, it is not a joke. Armed, uniformed, active duty military crossing a border is invasion. Even if they "just got lost".

I will admit, though, north korea isn't stupid. When faced with strategic superiority, the tactically correct decision is probing skirmishes (see above) and suicide attacks (https://en.wikipedia.org/wiki/1996_Gangneung_submarine_infiltration_incident).

The strategically correct response is containment with overwhelming firepower. That's where NK is now. Sucks for them.