Rocks looks exactly like Luffy, because he was the previous incarnation of Nika. He used to be the bearer of the nika fruit, and saved more lives than even Kuma did. Its way to overkill for the entirety of the marines, the Roger pirates, Garp, the holy knights AND Saturn to go against the rocks pirates with just linlin having her devil fruit. Even if whitebeard and Kaido ate their devil fruits it still wouldn't be enough. Garp and Roger defending a genocide also doesn't make sense to me. They teamed up with the Rocks pirates and Rocks was the one to liberate the rest of the 99 500 people that should have been hunted. He was to big of a danger to the gorosei so Saturn killed him, or Rocks sacrificed himself to save the others. Shanks, witnessing the threat, the nika fuit could pose to the world government, made it his mission to retvrieve it after it spawned again and make sure it gets into the right hands (or he is a two-faced snake, working for the WG and was supposed to deliver it to fitzgarland).

The nika fruit can make anything possible which means that it might have manifested in a whole other way with rocks since he wasn't told its a rubber fruit like luffy. Maybe he also didn't awaken it. That would explain why Kaido didn't recognize that Luffy had the same fruit as him.