It means you dont need sharp tools to leave scars. You just need to punch harder. Which means Yamato aint hitting hard enough, that's why only a drop of blood from Kaido with her best attack.
luffy literally tore apart Saturn and didnt leave a scar.... thats about 1553453653x more than what 99.9% of the verse could do to kaido.... so a feat in Yamatos favor.
neither luffy nor katakuri got scars from having half their torso ripped to shreds
scars dont have any logic in OP theres been like 3 prominent scars in current timeline and 2 are on Kaido and 1 is on Shanks