It's not like the forehead is a brick wall that won't give at all. Sure, it's hard, but the human head only weighs ~10 lbs, so it's going to snap back when you punch it. So yeah, I'd buy it that he can punch a guy in the forehead and not feel it (much) without being a mutant.
I have mixed opinions of makiwara. I was told to hit it as hard as possible and the knuckles would wind up breaking, but would grow back stronger. (Maybe the fact the guy telling me this had one huge knuckle on each hand where his first two knuckles were supposed to be should have served as a warning.) That was fine until I got a painful lump (that I could poke and move around under the skin) on my left kunckles that wouldn't go away, even after I stopped makiwara-ing. It gradually shrank a bit, but didn't go away and stayed on my hand for years. It only finally went away when I wound up taking a complete hiatus for ~2 years.
Nowadays, I just do weight/pressure and not shock/impact for knuckle conditioning. If I ever feel like I need more, I would go about it a lot differently and do a much more gradual buildup of how hard I hit the board.