I am quite sure that was Kaido's strongest move, plus with that size I doubt he can easily dodge Bajrang gun.
With that size he dodged Snakeman Black Mamba that is way faster than Luffy's punch. And the point is that he shouldn't be that size and shouldn't leave Onigashima so Luffy would never land the hit.
But the real main point is that Kaido didn't used any attack. He used a transformation and was never told to be the strongest. It lacks feat. It is similar to Luffy Tankman but I think even Tankman used a counter attack on Cracker if I'm not wrong.
Just recap all fights of Luffy. Majority of them villains are using their strongest moves at the end but we already know they are the strongest because they used before.
Kuro invisibility attack. Arlong biting Luffy. Lucci Roukougan. Moria using thousand shadows. Hody shark water attacks. Doffy awakening. Karakuri club punch.
Exceptions are Krieg that simple wanted to dive Luffy on water. Crocodile that used sand and Luffy was no longer affected by it. Enel similar to Crocodile. Caesar who was hopeless against Luffy. And finally Kaido that didn't attacked him.
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kaido was over confident but stupid
just like doffy in DR
Enel prets
list goes on
Fake. Doffy tries to use the whole country against alliance so he didn't had to fight. He separated Luffy and Law so he could win. He even kept calling Luffy a animal or something like that showing how aware he was of his capability.
Enel was just the same he tried to make a competition so only 5 would last with him. Then against Luffy he adapted himself because lighting wasn't work on him.
Now Kaido. He already had a big army that they have to defeatd and move forward until finding him. Then he is adapting and using stronger forms and moves to match the opponent skill/power level.
I would say that Doffy was the only one not overconfident but none of them was stupid. Oda made a stupid decision with Kaido and Enel last attacks.