He needed to hype up the current arc villain. Every arc villain is more or less the strongest, perfect, undefeated warrior until he meets Luffy. Just like Katakuri was the perfect human whose back hadn't touched the ground since birth.
Arlong: The strongest pirate in the entire East Blue, even feared more than "Bounty Hunter" Zoro
Crocodile: Luffy lost 2 times against him and even made his crewmates doubting him
Enel: Treated as a god throughout the whole arc, fodderized anyone, even Wyper and Zoro
Lucci : Strongest CP9 agent history, hyped up by everyone and their mother at Water 7/Ennies Lobby and even beyond that
Doffy: Had Law shit his pants and several flashbacks dedicated to portraying him as the mighty Usupator
Thing is, unlike with the others, where Oda delivered but hasn't gone overboard that much with hyping them up prior to that, he's blown things completely out of propotion for Kaido.
He hyped him up as the arguably strongest being and left us with hostage diffing Oden, hiding in Wano for decades and not a single feat or statement coming outside from Wano, from a reliable source to back up the hype.
The times he was captured could have easily been written in a way to hype him up, but it was never brought up against after his introduction..
Even Kaido trying to join the Marineford war wasn't leaving him in a good light at all, when he was stopped by Shanks. Obviously there is more to it since Shanks wasn't even scratched, but it's just another missed opportunity, ultimately leaving us readers with mixed feelings instead of the feeling of "wow, this man surely is the strongest being"