Kinemon can darken his sword with CoA:
But Kinemon didn’t use sword hardening to fight Kaido:
Darken the sword or not, it doesn't make any difference, Oda is inconsistent with that.
If Zoro uses him to defeat characters like Pica and Kamazou, then it is obvious that he was using his CoA's best performance to hurt Kaido.
The scabbards managed to injure Kaido, their swords were penetrating Kaido's skin .. Since the first attack.
while Zoro and Killer were not capable of that, it just means that Zoro and Killer’s CoA are still not good enough to overcome Kaido’s skin defense, and Zoro is training for this, using the power of enma