I suppose our opinions differ here, but I generally do not see it that way.
Zoro was cutting Kaido without CoC, we saw that with the Dragon twister. It doesn't make sense to show Luffy and Kaido using ACoC (via visuals too) and not do the same with Zoro.
The scabbards could pierce Kaido's skill with Ryuo, so I don't think ACoC is the only way to scar or wound Kaido, it's definitely the most effective way though.
I think if Zoro had hit Kaido with a CoC attack it would have much more of an impact on him.
I lean more on the side of Kaido sensed the haki leak out, but Zoro didn't actually use it in the attack.
We know this can happen, as seen with Ulti and Luffy where he was in the rubble and she could still sense his Haki.
I don't think Zoro truly gave everything to Enma till he fought King which then allowed him to draw out his ACoC, up until then he was still holding back somewhat as he alluded himself.
They do. We can just agree to disagree on this one but I'll still tell you my opinion on those points. For healthy discussion.
I believe Zoro already put but traces of his CoC in that dragon twister just like in the attack that missed during the early part of the fight, which is why his sword blackened with an aura in my opinion and why Big Mom reacted like that to it (inb4 some other commenters: no, that was not Oden's haki, Enma just forcefully pulls out more haki that Zoro intends to, that's all it does).
The scabbards did indeed injure Kaido but, as Kaido said: "their CoA was not enough to even open Oden's old scar"... with a simultaneous ultimate attack of the 7 scabbards right on top of his old scar. To me, that tells me something more powerful than just CoA is required, and I believe we especifically saw Oden use CoC to make that scar in the flashback.
It's possible that Kaido sensed CoC despite it not being on the attack, but while it's possible, I'll try to argue I don't believe it's likely:
- The literal same thing happened to him again against King. He accidentally used it on King when he poured through Enma by trying to "go all out" with it. Despite knowing nothing about CoC, he just poured it into the sword at the time of his attack.
- Just like he said he would stop holding back against King, stop trying to preserve his haki, his last attack against Kaido was one in which he admittedly put all he had left, mostly because he was on his last leg and he stopped caring about holding back. He didn't even know what he did, because he just "poured all he had into his sword", and that included something he didn't know he had.
- It would be too much of a coincidence, in my honest opinion, if pouring his all against King imbued CoC but pouring his all against Kaido minutes earlier just made his CoA stronger but coincidentally he also released a CoC wave for the first time.
The difference in the fight with King, is that he wasn't as half dead and wasn't going for a single all out attack when he decided to "go all out" on his haki, which allowed him to properly use CoC for a prolongued time instead of a single instant.
I do acknowledge that black streaks were not drawn in his attack on the rooftop, but this doesn't outweight the reasons I have to believe he used aCoC for a single moment. I have been arguing it since that very chapter, and was pretty excited when the King made an actual repeat and continuation of what I argued (sadly, I was off the forum at the time of that fight).
I'm surprised I'm pretty much arguing that I believe Zoro is weaker without his aCoC than you (non-specific plural) believe, me being a Zoro fanboy and all.