The skull is nearly an inch thick of aluminum-like calcium. People break their knuckles all the time fighting in gloves. Gloves also slide off the skin a lot better, so it reduces cuts.
This "homage to the old days of fighting" is ridiculous. Gloves have been worn for all modern day combat once gladiator fighting was frowned upon 500 years ago. It's a novelty because guys get cut easily so it's bloody.
Trying hitting a stationary heavy bag bear knuckle. Put a layer of wood over the top of it and that would probably be similar to catching the forehead of the skull when throwing a punch. For the sake of not needing hand surgery and your opponent not needing stitches at the hospital (or vice versa), let's just stick to gloved fighting. We still get all the knockouts.