Maybe I am just presenting this in a weird way, so let's try this.
Lunarian DNA makes it so King's skin is extremely hard to cut through.
Wapol Metal sits UNDER the skin.
Reinforcing a Seraphim with Wapol Metal would make it so that in the off chance someone has the potency to make it through the Seraphims skin, even if slightly, the Wapol Metal would stop the attack completely.
So when Zoro slightly cut King, and made him bleed, even though his defenses were up, if King had Wapol Metal under his skin, he wouldn't have bled at all.
The Lunarian DNA would have stopped the bulk of the attack, and the Wapol Metal would provided extra protection to stop the rest of it.
It's two layers of defense before you get to the blood and bones.
So for example, say a mountain level attack hits the skin and breaks it, by the time it hits the Wapol Metal its only house level. Without the Wapol Metal, there is potential for much greater injury.