Comparing Luffy and Kizaru here is a false equivalence.
Luffy has already shown and had both CoC and ACoC explicitly confirmed during his fight with Kaido. So if he didn’t show the usual ACoC effects in Egghead, we can simply say he wasn’t using it at that moment, because we already know for a fact that he has it.
Kizaru’s case is completely different. After Sabaody, Marineford, and an entire fight against a Yonko in Egghead, he still hasn’t shown even basic CoC. He has never knocked out fodder with it, no character has ever acknowledged him as a Conqueror’s Haki user, there has never been any confirmation whatsoever, and he definitely hasn’t shown the advanced coating.
Saying, “Kizaru might have just chosen not to use it too,” is circular reasoning, because you would first have to prove that he even has it. You can’t use the fact that he has never shown it as proof that he secretly has it but simply chose to hide it.
Luffy not using an ability he is already confirmed to have is a choice.
Kizaru not using an ability he has never once been shown to possess is simply a lack of evidence.
And claiming that Kizaru has ACoC is even more ridiculous, because before giving him the advanced version, you would first have to prove that he has basic CoC in the first place.
Could Oda reveal in the future that Kizaru has it? Sure. But a future possibility is not current evidence. Based on everything the manga has shown so far, Kizaru does not have CoC or ACoC.