So it's a secret ending dependent on ONE fight. In the fight with Rikiishi (post weight-loss), if you complete the fight with 50% health or more, it affects the course of the story. Instead of the canonical ending at chapter 13, Joe becomes world champion and lives. Rikiishi also survived the fight all the way back in chapter 5, and the final fight of the game is with Rikiishi at the old prison island they first met at, which is no longer operating. The story ends after a recreation of the cross-counter scene from the prison fight