Striking DR 3+ Tough Skin without Hurting Yourself?

[THN]

I made my first Dungeon Fantasy Barbarian under the Boxed Set rules.
After buying Tough Skin 3, I noticed the Hurting Yourself rules on the Exploits booklet (p.40) don't exclude this kind of Damage Resistance when striking DR 3+ armor.
The Tough Skin description on the Adventurers booklet (p.16) doesn't talk about it, either.
I searched the Boxed Set PDFs, just in case the Tough Skin exception from B379 was hidden somewhere else, but I couldn't find it.
I know many subtle tweaks have been made to improve Dungeon Fantasy and make it more accessible to new players, so I'm not sure if this is intentional or an overlook.