But he hasnt closed the gap to have 4 or 5 billion. Kaido has fought many people who have weakened him.
We can see ACoC as a sword and without it, a naked hand. Once you have ACoC or the sword, you can already pierce or cut anyone, but your ability with it is still not top, but the power it already is. Besides, concetrating your haki to your attacks is something Luffy already learned with AdCoA, so it is not bad if he could apply the same with ACoC.
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Mmmm Kaido directly said that Shanks could challenge him lol, how could he know that if he didnt fought him?
BTW, in his introduction, it was said he fought all the emperors.