Something like this is also very likely. Haki depletion has been a thing for a while and since Luffy hadn't used CoC this way before and didn't know his limits he just overused it rapidly.
Reading between the lines I thought it was more of a "fancy new trick you've got there but hardly enough to tango with the big boyz, amateur" kinda thing where he was alluding that there was never any way Luffy was going to get anywhere against him, with or without CoC.
It's obvious CoC will be part of the answer to defeating Kaidou or any yonkou for that matter but it alone is not enough. What else is there though? Fruit awakening? shadows?