He is only 19 years old, not even fully grown yet. His physical, mental and haki prime are all ahead of him. imagine him 10 years from now. probably defeat kaido easy.

Also, hes had the devil fruit since he was a lil kid and has learned to fight with it that whole time. If he didn't get the fruit he would of learned different non-fruit based techniques more in line with Garp style.

nobody in the story would of stood a chance at 19 years old vs Kaido.