Looks like Luffy realized AdvCoC is useless if he can't tag Kizaru, it would only waste more of his limited haki during G4?
Look at the panel posted above by Kurwa, Luffy was sure he could land blows on Kaidou, and Kaidou had super durable body so advCoC was the way to go. Seems Luffy wants to be sure he can even land an attack on Kizaru before wasting more haki, and in the offchance he lands advCoA is plenty deep to hurt a non Kaidou opponent, especially if they rack up.
He did the same with Katakuri, where he wasn't bothering to go G4 without testing the waters, because he couldn't land shit either way.
When I read the spoiler, I initially thought he would be able to go back and forth with Kizaru in Snakeman, never would have guessed Kizaru dogged Snakeman that badly, now it makes a lot more sense why Luffy does not want to use more excessive haki without confirming he can land clean.