I used to doubt Kaido's title myself but the thing that convinced me he is really the strongest is Oda's reply in the SBS
In a question where it was asked if a mother was stronger than Akainu, Oda used his title to justify a mother is even stronger than Kaido, let alone Akainu
That was the moment i was sure his title was legit. With that said, i have an exception to that title. It's Blackbeard. Unlike the other top tiers, he is in the worst generation, who keeps improving, was already a yonko a year ago.
I suspect he grew even stronger with an additional year, but just like when people weren't accepting him as a yonko until he overwhelmed Marco, i believe Oda is waiting for Blackbeard next demonstration of power to show he is actually the strongest character currently, but since it's speculative as of yet, i place him in a tie position with Kaido right now