Aight, I figured it all out, no thanks to the rest of you cretins.
Yamato isn’t real. He’s a shared hallucination that Luffy/Kaido/everyone else in the story is having, and they can’t figure out why. All of them are stunned when his name comes out of their mouth, because they don’t remember saying it and can’t picture the person its attached to; when they see his face, they recognize his looks but cannot place him in their mind.
This will come to bear in the middle of the final conflict, when Luffy and Kaido will both scream his name before their final blows land...and they will stop mid-motion, eminently confused and heartbroken over the cold, final realization that he never was, and never will be. They will both become inconsolable and embrace for a long time, leaving all other combatants uncertain of how to proceed before laying down their arms and learning the real lesson - life isn’t about winning, it’s about having a make believe child/friend who brings you closer together with the ones you love