He doesn't know that he has CoC, it's either he unlocked it now (unconscious use) or Asura is manifestation of CoC, which he can't know.
The first letter can´t be since Zoro know CoC and he even answer Brook in chapter 997 that he didn´t use CoC.
Zoro have well knowledge about CoC, so he would know iif he have the first CoC step.
With means Ashura beeing the CoC source and Zoro don´t knowing it make more sense, look how Kaido said it:
Kaido is struck by Zoro's attack with lot of blood coming out.
Kaido: "Brat... Don't tell me you have the "color of the supreme king" too!!?"
Zoro: "What the hell are you talking about... I have no idea myself..."
Color of the supreme King never get used for a attack, but Kaido take a hit and then release Zoro using CoC.
And in fact Kaido get hit also with tatsumaki which don´t even make any comment about Zoro CoC means that Zoro source of CoC is Ashura, because that Kaido feel the CoC AFTER he take the attack. With means Zoro put CoC in his attack, hell the key of the chapter is about CoC. It was the way how Oda want to tell that Zoro unlock adv CoC(manifest) with Ashura.
I mean Kaido never comment about Luffy using CoC in his attack before this chapter coming out, but when Zoro hit him he feel it?
Does this mean Zoro who unlock base CoC have more power then Luffy in CoC?? That doesn´t make anysense, so the only answer is that the source of the haki was Ashura, it was so strong that Kaido feel it.