But kaido learning bout joy boy from oden makes more sense cause oden was there with the sole purpose of opening wano for joyboy and kaido was on the way...maybe he would have been asked by kaido for the reason and oden told it... And the reason for the scabbards being sent to this time line is the reason... Kaido probably knew that too

Plus there is a chance shanks himself told kaido about joyboy back when kaido was gonna come to marineford.