Only Joyboy can beat him.
That's why Kaido said so, he has no idea how strong Joyboy would be or what power he could posses.
But he knows Joyboy would be that type of person who would beat him, the person that can change the world.
None of those 5 people are Joyboy. And none of them can change the world either, that's why they were making way for Joyboy in the future. Shanks, WB, Roger, Oden were all waiting for that person and make way for that Joyboy person(Luffy)
Kaido is the gateway or the boss which the person who could beat him will be the one who can change the world, and that's one and only Joyboy aka fate boy Luffy.
It's important to know Oda make Kaido this strong.
Strongest individual character ever in my eyes with utmost due respect to Roger, WB etc