There’s a difference between durability and holding an edge. Sure the katana might not bend or get broken easily, but it’ll still dull quickly when used to cut through bones on a regular basis (You just decapitated 100 zombies, your katana won’t be very sharp anymore unless it’s made out of adamantium), and trying to one hit kill using a slashing weapon is highly dependent on how sharp it is.
Yea, that’s why it wouldn’t just break, but having a loose sledge wouldn’t hit as hard, it would both move and vibrate, making it harder to hold on to and impact with less force.