After looking at this panel for 10 seconds I had an epiphany!
Kaido says that a CoC user needs to already BE the strongest to use CoC coating, it's not just about skill and flowing CoC haki, there's also a power level requirement that's gatekeeping Midd, Mingo and Katakuri from ever having it, you need to be the strongest to get there and only then comes the skill to use it, Kata and Mingo are very skilled but they're weak as fuck and Midd can barely use haki, even if an Admiral had base CoC he would be weaker than Kaido just using CoA.
Tldr: The very strongest are the strongest even without using CoC coating and their strength surpasses every basic CoC user in the manga.