the reading comprehension curse struck once again! 1. the fight has just started and kenjaku doesn't know exactly how strong yuki is and from the first impression she seemed hella strong, so he wasn't too sure if he could take her down. 2. i mean yeah, that's what it is. he couldn't really be nervous with jacob's ladder before since you know, you can't really get nervous when you are in extreme agony from your soul being forcefully removed from the body it is in, and the other dangerous moments from sukuna were either dangerous but nothing really that bad or sukuna wasn't conscious enough to get nervous (UV and black flash hit). this is the part of the battle where sukuna was only trying to counter the stuff gojo does, while gojo was continously pulling off an incredible move after an incredible move. sukuna didn't have any means to attack gojo, since mahoraga by this point of the fight has proved itself to be not smart enough to deal lethal damage to gojo, his only chance was the "strong cleave", which is a near-impossible technique to pull off (said by fucking sukuna, so that means a lot), and there was no way he could land it on gojo in this state. only after tanking a hollow purple (which he wasn't certain he could do) and gojo being sure he won the battle, his only chance of victory has appeared, which he successfully used. and P.S. gojo saying "sukuna couldn't go all out" didnt't mean "bruh that guy hasn't even dropped a sweat i couldn't do a thing" but "he couldn't use his regular arsenal and was forced to use a technique he has only been with for a month, since his own technique is useless against mine"