For the sake of transparency, according to MShadows who lives in Japan and is fluent, Zoro refers to Kaido as the World's Strongest in a sort of mocking way:
Zoro: I have come here to cut down the so called “strongest in the world” Kaido!!!
Wow just Wow.
I am speechless, Zoro ususally doest act like this but after he had this moment of reflection watching Kiku arms falling from the sky (ignoring everyone in the field) he get to the conclusion that Kaido is a piece of shit and need to be dealt. This is huge guys!!!