Kaidou was yelling when Rocks hit him. There is no proof Luffy took significant damage but even if he did it is already established Nika's durability > Kaidou's when he treated some of Kaidou's strongest advCoC attacks as memes. So whatever damage Luffy recieves will damage Kaidou that much more.
You can't use someone with higher durability than Kaidou to then say Kaidou wouldn't take damage, makes zero sense.
That's not how it works
Kaidos fodder attacks were ripping g5 apart
We don't know how kaido takes his baguas
We do know from the fodder slashes
Odens paradise totska one shots g5
Yelling means nothing
We know
Anything less than ryou = no damage
Anything less than acoc = shallow damage
This is not the case for luffy.