Yes exactly, from within, how is that supposed to leave a scar? He draws blood from kaido's mouth constantly when he punches him, it's clearly doing damage and that's why he won. He can't leave a scar or cuts with his fists.
You don't get it. Understand the type of attack Luffy has and how it works.
Luffy possess Internal destruction haki. That power breaks/destroys objects from inside out. Remember what Luffy did to the tree. When his internal destruction haki attack hit the tree it broke it into pieces from inside out.
Since Luffy punches have internal destruction haki why aren't his punches destroying Kaido's scale or broke it into pieces and causing scars? If that attack hit your leg part of it would burst open and leave a bad scar.
If Luffy attacks aren't causing that type of damage and scarring Kaido's body that means his AP isn't strong enough. Not even cutting attack scar Kaido.