So, no headache; just a a bit of blunt force trauma that he/she would have shaken off very quickly?
Betcha this piggy had constant migraines for a while! Shows how darned tough these animals really are!
As 'Redchevy' noted...IF that broadhead is as far back as it appears, then it would have completely missed the brain-pan of the skull.
If you look at this Hog skull I bisected....you will see that there an area to the rear of the skull (above and behind the brain pan) that is nothing but porous bone.
I don't think anyone can know for certain what the hog experienced. Logic dictates the animal might have suffered a 'shock' from the blow, possibly even 'disorienting' for a short period of time, but it clearly was not lethal wound. I am more surprised the wound site didn't cause a debilitating infection.
I've killed a number of hogs missing large parts of a leg, so perhaps their immune system is hardier than we might know.
I have to wonder how long the arrow 'shaft' remained in place? Did it immediately break off at the broadhead or the did the hog go around looking like a unicorn for awhile?