If Sanji were to unlock COC then it should be the next chapter (that focuses on Sanji v Queen gain)
I'd say "if" it truly happens, I think it'll be at the absolute end of the fight where Queen himself is shocked because it he (possibly) doesn't have it. Sanji won't know what it is, ala Zoro vs Kaido. Then Zoro vs King, we'll see Zoro use it in a conquerors clash with King and the use it again when he beats King.
it's a good way to avoid keeping CoC usage redundant while also allowing both Zoro and Sanji to grow at their own pace. As of now Zoro has it, so it's only fair he elevates it's use. As of now, Sanji doesn't, so its fair that he only reveals it inadvertently or just doesn't here.