The relevant rule from standard chess:

> Leaving one’s own king under attack, exposing one’s own king to attack and also ’capturing’ the opponent’s king are not allowed.

N6 and K8 both expose the black king to attack, so black is not allowed to make those moves. And with no other options, black has no legal move.

And since black isn't in check where they are right now – that's a stalemate.