Well tbh I think Sanji will unlock COC eventually, but still I cant think of any scenario epic enough for him to unlock it now (like Zoro's Asura or Luffy vs the Boa sister).
I mean, tbh, i don't really think those were the most epic scenarios ever.
Zoro's on paper could have been, but the presentation and focus dedicated to that scene, with even the art to support it somehow wasn't there. Although like i said, it had the perfect premise to be there when you just put forth the idea that he will unlock CoC in the process of giving Kaido a scar with the first ever Ashura move he uses post time skip.
Somehow that sounds way better described than it was delivered. Hell, we would not even know has CoC if Kaido did not make a comment on it in one panel after the fact.