Yes that's been told couple times.
Kaido himself was waiting for the guy for the longest time, no one else.......
He's a pirate with having the most combat experience in life......from being a member of Rocks crew witnessing some of the strongest pirate in history like Rocks, WB, Shiki, BM etc......being in God Valley having to experience the battle of highest tier which includes another big names like Prime Garp and Roger in addition to Rocks crew
Then finally battled Joyboy Sun God Nika
He meet pretty much all the strongest character possible. From Rocks to Roger to Nika. That's one hell of a pirate career for a guy who's searching for someone strong.
And among all top tiers alive he doesn't think anyone other than Joyboy could beat him. And it happened.
I'd pick his words(author's word) over any mere fanbase agenda.
Luffy < Kaido (before Gear 5), he needed the help of people to stay and come back in the fight until he awakened
"You couldn't be JoyBoy either it seems"
However, it is factual that Nika >= Kaido (if Nika = 100, Kaido = 99.5)
I don't care what agenda people say, I will listen to the author
The author clearly said through Kaido "JoyBoy is the one who'll be able to beat me"
Kaido knows that nobody could defeat him but JoyBoy
Even King tells him "I suppose we'll never see him" because he doubts that anyone can even beat the world's strongest creature
Kaido couldn't be defeated by anyone before Gear 5 because he was the world's strongest creature
Nika was the only one who could defeat Kaido
Both Nika and Kaido are the strongest pirates
The problem with Blackbeard is not that he will be stronger than Nika but he will cancel Luffy's Devil Fruit and so, he may not be able to fight in Gear 5 all the time