I don’t get this one. Why should NPCs get invisible armour? Because the AI sucks so much they don’t know how to dodge and use cover? Imagine the frustration knowing that you have to hit an NPC 20x before they will take any damage.
No - I think he was pumped with adrenaline and wouldn’t have noticed it much unless it shattered the bone and impeded his ability to grip the saddle and/or use the stirrups. Just because there is no backstory negating any horse riding experience doesn’t mean he was a total novice. Had he been one, he would have struggled to even mount the horse (i.e. you have to hold the reins with a small amount of tension on the bit to prevent the horse from taking off). Let alone a horse that is not war trained and would have been immensely skittish with all the screaming and mayhem around.