That he became stronger lol, as anyone.
What difference was there between East blue Zoro to Sabaody Zoro?
Kaido sure as hell used haki, as all top tiers, but his main focus has alwasy been the fruit. His deafening feature is about the fruit:
There are a million of these, but let's stop with these two.
Even his final attack was a bigger fire dragon. Obviously with CoC, just as Luffy's final attack was a G5 (fruit) attack with an island sized punch, with CoC lol.
And then proceeded to think of himself as the WSC even when Shanks, whom he met, had a stronger haki.
Kaido never said that having a stronger haki meant all, and anyone he himself was the proof of the opposite. And anyway he literally didn't know the first thing about how the real world worked, as we are seeing.
Perfect.