Joy Boy final battle was not 1v1 but against different founding Kings/Queens. There were originally 20 Kings/Queens but I don’t think they all fought Joy Boy. IMU being tied to the number 16 makes me think it was 16 of the Kings/Queens who fought Joy Boy, making it the 16 Holy Bullets that defeated Joy Boy.
Looking at the WG, there are 5 Gorosei and 10 Holy Knights + IMU, making 16. Luffy fighting these 16 gauntlet style would mirror Joy Boy’s battle. Oda also foreshadowed this with Kaidou the WSC and failed Joy Boy who fought, but ultimately lost to 16 individuals, 9 Scabbards, 5 Supernova, Yamato, and Momosuke. Luffy is the only one who can succeed in this gauntlet and surpass Joy Boy.
Looking at the WG, there are 5 Gorosei and 10 Holy Knights + IMU, making 16. Luffy fighting these 16 gauntlet style would mirror Joy Boy’s battle. Oda also foreshadowed this with Kaidou the WSC and failed Joy Boy who fought, but ultimately lost to 16 individuals, 9 Scabbards, 5 Supernova, Yamato, and Momosuke. Luffy is the only one who can succeed in this gauntlet and surpass Joy Boy.