In KSS, there is an ability called Mirror. As you can probably guess, it reflects attacks.
Also, in KSS, pressing R creates a sheild. Some abilities have unique ones, for example, the Ice ability freezes kirby.
The Mirror ability creates a Mirror Sheild, which reflects EVERY attack in the game. Except one: Dedede's Hammer Attack.
Odd, as all of Marx's attacks are reflected, as well as Meta-Knight's. You don't take any damage when reflecting, by the way.
So why does D-Hammer pierce through the shield? My theories are:
1. The attack can pierce shields.
2. Dedede swings harder when you guard, more powerful than Marx's Black Hole attack. (M-Black Hole does 50% damage to a full health bar)