I don't know why people get so hung up on whether aCoC was used or not, when it seems Oda himself isn't too stressed about it.
Me personally, I feel that the prior CoC clashes at bare minimum effected both users in a test of wills that would leave one at a disadvantage to the other. The effects weren't advanced so they were AoEs that had a lot of leakage, so naturally aCoC would be the AoE effect concentrated into a weapon/body part. Otherwise why not just reserve the haki stamina required?
It's not even like it has defined effects since Zoro creates green flames in addition to AP while Luffy just hits significantly harder. Personally I feel Kaido's drunken techniques incorporated not only his fruit, but aCoC for its weird effects. Who knows maybe its even nerfed nen for One Piece