It should be against Kaido, and would make no sense whatsoever if it doesn't happen. Luffy has had a Conqueror's Haki clash with every CoC user he has faced, so there is no reason why he won't have a Conqueror's Haki clash with Kaido. And considering how Kaido and Big Mom altered the weather with their clash, it stands to reason Kaido's CoC is top notch. If these two clash with CoC (Which they will), then it has to split the sky. There's just no other way around it.