Kid said it probably did damage, Luffy is here saying all his attacks are doing damage to Kaido.
Kaido one shotted Oden, that doesn't mean he was far above, it meant it was plot relevant, similarly Luffy was plot nerfed due to be begining of the arc (Croc, Lucci, Caesar Clown, Monet, all people not an untouchable distance above Luffy and sometimes weaker who gave him begining of the arc Ls), nerfed due to the fact he could not bypass scale defense and enraged and confused so FS didn't work.
Luffy isn't using his full trump cards either. Kaido got ragdolled and had to wait for G4 to run out, those are the facts.