So....
For CoC Coating, you have to use Ryuo with CoC instead of CoA
Luffy learned Ryuo and trained it for 2 weeks.
Then, against Kaido, he realised that you could use CoC instead of CoA when doing Ryuo.
So he figured out CoC coating.
Zoro is the reverse.
He has had Ryuo this whole time, but he didn't know about his CoC (my guess)
So now, he figured that he has CoC and is using that in Ryuo instead of CoA.
Does this make sense?