Hogs armor is a myth. Sorry.
No, it is very real and some of what it can do is real. However, there is a lot of myth associated with armor. The armor has been shown to stop some birdshot, buckshot, and .22 rounds (pellets and slugs found in the armor by later hunters). The armor, more prevalent in males than females, is a layer of cartilaginous tissue which is fairly tough. Contrary to myth, it is not made from scar tissue and is not caused by fighting or rubbing. It is genetic as it is on some other "armored" animals like rhinos. It is claimed that it is there to protect the vital from injury from other hogs, but the vital tend to be lower than much of the armor shield and there is none low that protects the neck or heart. Given the amount of armor shield over the shoulder which is heavily muscled, and along the top side of the back, it would appear that the shield is in place to protect the animal from insults such as tusk stabs, but not necessarily of the vitals, but of other structures such as the shoulder girdle where most of the high strike tusk stabs occur (versus the low strikes where there is no shield and that are close to the heart and lungs) and where there is already a lot of bone and muscle "protecting" the internals.
I would posit that the shield is commonly blamed for a hog not stopped when the real culprit is along the lines of poor ammo choice, poor hunting/shooting skills, etc. The shield may yield a couple of inches of extra tissue through which the bullet has to pass (often much less), but that should be no problem for most proper ammo.