I just realize that in this picture the koka on Zoro’s shoulder is thick it is like he is wearing an amor on top of his skin, like Katakuri’s CoA:
Look how at Zoro haki when he used Enma for the first time:
As you can see there is less « density ». The CoA seems to be a part of his skin not to be a top of it (like Luffy’s CoA for example).
It seems that the Enma training allows Zoro to reach Katakuri’s CoA level:zorothink: