Kid went into the New World without training and look what happened. He lost an arm, he has many scars, his reputation is in tatters (Caribou and Brûlée prove this), and his best friend Killer has eaten a failed SMILE fruit. Even the fanbase constantly makes fun of Kid for being a loser.
The other Supernovas have fallen under a Yonko or have been biding time like Law.
Luffy on the other hand has only been in New World for months and already has a 1.5 million berry bounty and a Grand Fleet. He has suffered no big losses to his crew and he has all his limbs intact lol. Kid only has a 470 million bounty and he’s been in New World for 2 years, but that does not mean that Luffy is superior to him it just means that Luffy was preparing for 2 years while Kid was struggling for 2 years and Luffy’s preparation paid off.
The New World is an unforgiving sea, so rushing in to many battles is a high risk high reward situation. The risk is severe injury or death, but the potential reward is exponential growth. We know that Kid threw himself in to many fights, lost an arm, has numerous scars and could’ve lost his life as well as his friends lives. But since he still lives, he’s likely reaped the rewards of that high risk situation and has grown significantly.
Luffy trained the safe way, Kid trained the risky way and we see the results. So yes it makes perfect sense for the Supernovas (mainly Kid) to be on Luffy’s level of individual strength.
Roger himself had Whitebeard that was equal to him, as well as Shiki and Big Mom who stood close behind.
*by ReggieZoldyk21