Kaido only told that to King...π
Kaido said to King that JoyBoy is the one who defeats him, we may as well assume that the knowledge of JoyBoy returning and defeating Kaido is widespread. So, let's say that everyone and their mother knows that JoyBoy is going to defeat Kaido, which actually happened, how can you prevent the world from knowing JoyBoy is back or at least, cover it partly? By making Kaido's defeat seem like a combined effort from 3 members of the worst generation (Luffy, Law, Kidd) so you can't easily say that it comes from a guy who became JoyBoy.