Luffy fought Kaido first time and didn't know about the scales. That's the point of Udon arc where he trained to get haki to break the scales.

Meanwhile these 2 dumbasses knew about the scales and still rushed to fight Kaido with nothing but gimmick of slamming around and cutting with a sound.
They literally admitted that they had no chance of victory before luffy punched Kaido
How would they know that some blue old man was a ryou master, and kid never got first hand training from the pirate king's right hand so he didnt see ryou.
 
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