Title.
Here's a summery/ timeline of events to help out people with minimal reading comprehension.
Zoro first cuts Kaido.
Kaido then inspects the wound.
After inspecting said wound, he exclaims in shock.
Only after he got cut did he mention Zoro has CoC too. This is because Zoro wasn't trying to knock out fodder, rather so the CoC was applied on his blades/ attack.
And confrims that it would leave a scar due to the nature of the attack (CoC)
From the above sequence of events, it was after inspecting his wound that Kaido realised that Zoro had COC.
Therefore, something about the wound Zoro left on Kaido made him realise that Zoro used COC.
Therefore, Zoro infused COC into his attack. It's like an early type of advanced CoC. A prototype/ failed version of the ones top tiers use.
It is what it is.