Well, the current Kaido only believes that Xebec is capable of fighting him, not defeating him. Does that mean anything to you?
Placed above both by King.
If Kaido expressed that Rocks D. Xebec, Gol D. Roger, and Edward Newgate (Whitebeard) are only capable of fighting him, it implies that he doesn't necessarily believe they are stronger than him, but rather that they are formidable opponents. Having the ability to fight someone doesn't automatically mean they are stronger.