Gojo in 15 chapters could not do what Kashimo could in one ..... And so. Sukunu, being not in the best shape, believed that this would be enough to defeat gojo and he won.... As soon as Kashimo turned on the cursed technique, Sukuna could not even react to Kashimo's speed, although before that he easily killed gojo, who was under the black lightning buff. Sukuna had to use true form to win. That is, Sukuna believed that Kashimo was more dangerous than Gojo at the time of activation of the cursed technique, otherwise he would have just cut Kashimo like Gojo.....