Unless we see a flashback of Bran telling her where to hide, and how to attack the NK.So if this is indeed the case, then Arya just got lucky that she stabbed at the right spot? And on top of that, the Night King's weak spot was one of the only spots on his abdomen that was not armored?
I think Valyrian steel just kills those fuckers and the showrunners were simply saying that they wanted him to get stabbed there so that there was some parallel or something between his creation and his death.
It will make Bran's walkabout make sense, and give it purpose.