It's not about the scars. It's about the significant damage. The scarring Kaido is how Oda showed, a character with an invincible body, received a significant amount of damage. The fact that is a feat only achieved by one other person and it happened 20 years ago makes it even more impressive.
If Oda monster lettuce know a character receive a significant amount of internal damage he will tell us in one way or another. For example, receiving 20 to 30 broken bones.
What is more impressive. Pushing over a thick steel beam or cutting through a thick steel beam with a sword with one slice?