Sukuna being in his True Form doesn't matter when He's so weak from gojo that he can't kill yuta and Yuji, who straight up would've die instantly if gojo hadn't weakened him so much
Also, he was toying with her to savor the fight (somethign he does with everyone) and only beginning to get serious in the fight, hitting her so hard with a black flash it takes her out of the fight for some time.
As for slashing feats, Sukuna could have his arm cracked by mahoraga (enhanced with cursed energy reinforcement) after he adapted to dismantles and could still slice through him. Ryu is relative to 16F sukuna and cpuld still be sliced into by a surpressed dismantle, later dying to a cleave to the head
"Reggie was able to stand on non-tangible, permeable shadow by reinforcing his feet with cursed energy." If reggie, the guy who isn't a fraction the Jujutsu Genius Sukuna is can do this, what do you think sukuna is doing? This ability is literally on the Cursed Energy Manipulation page, I'm not saying anything he doesn't have
Weakened Alastor never uses his barrier in a fight and Weakened Alastor has nor barrier feats. Given how Sukuna fires dozens of these and they're at least as strong as sukuna, he tears through it like butter
So does Sukuna and Alastor, while benefiting his survivability, will also be a detriment as Sukuna will see that Alastor is durable and can regenerate and will pull out his domain expansion, just like he did for the finger bearer, someone far weaker than himself.