Because cutting fire is just a technique.
The scabbard are allowed to be able to hurt Kaido. I said the amount of ACTUAL DAMAGE they do will be AT BEST one attack from Oden level.
At best, the scabbards will leave Kaido as damaged as he was 20 years ago. But remember, After Kaido took that amount if damage from Oden, he still had the strength to One shot him.
I should repeat, Kin'emon cutting fire is as bug a feat as That old man using the barrier fruit to block Oden from killing Orochi. We don't wank that old man for blocking a top tier right? Because his ability is specifically to block ANYTHING. And kin'emon says his ability is to cut ANY FIRE. SO HE DID THAT.