I have pointed this out earlier, but Kaido's affiliation to Wano goes beyond him just wanting to establish a weapons factory or gaining a territory which is a natural fortress by itself. A lot of his dialogue has pointed towards him knowing a lot more about Wano's past which extends beyond his enmity to Oden
Perhaps his "oni" race or whatever it is had its roots going way back to Wano in it's past and the persecution angle stems from there. The existence of Onigashima is probably also a hint towards that.
There are going to be 3 different flashbacks in this arc- Kaido's, a bit of void century through Wano's lens and Zoro's. The one giving us context to Wano's past could foreshadow more on legends like Imu and Joyboy.
I'm also curious to see how exactly Xebec would be incorporated in Kaido's flashback.