-it marks zoro's power limit before he unlocks power up. and speaks of his terrifying potential.
-it is an external attack, zoro is the only one in onigashima to do that, compared to the rest who aren't strong enough to do it instead they rely on tricks that bypass external defences like shockwaves/soundwaves/internal destruction.
-zoro inflicted it on kaido under an excruciating physical condition, a scar that kaido can't heal from and serves as a constant nerf for the rest of the fight, unlike the shallow damage that kaido can recover from instantly.
after zoro unlocked AdvCoC it's confirmed that he can slay kaido with one swing
scarring kaido is the same feat which oden performed , even kaido never mentioned the scar in the flashback but then he got ptsd for 20 years , oden scarred kaido in his weakest form whereas zoro scarred a much stronger kaido and that too even in his strongest form , if this was kaido when the scabbards failed to open a scar , then zoro would have done much worse to a dragon - mode kaido considering he gave him a brand new scar .
but again why will oda hype up zoro scarring kaido when it will be over-shadowed by him killing kaido .