It could be a blade that is easy to conceal. If we definitively called this a knife yes, but there is nothing wrong with pointing out there is an unknown object in the assailants hand. Even if it is something innocent, it adds to the defense of the shooter when added to the other facts. It allows defense attorneys addition ground to claim the defendant had a reasonable belief that his life was threatened as he may not know what the object is either. Not that there is a shortage of that evidence with the assailant attacking the victim's head with a blunt object.