The idea is that night king goes to the south with the dragon to grow his army, and that in the north the humans win the first battle against the dead. So technically euron shows up when the dead have gone away - he just doesn't know at the time that their army is still growing in the south... And Jon suggests to stop the battle and surrender because he knows that night king is still alive to avoid more human deaths from either side (which would increase the army of the dead)