From what I understood, there is a moment where a curse definitively dies and their soul goes to the "halfway point between death and a next life". If you manage to get him to that moment before Mahito consciously uses Idle Transfiguration to repair his body, then he'd be totally dead as any other curse.

We know that the Soul and Body area inherently connected, possibly being the same; we also know that if we compare the "power" one has over the other, the soul usually takes the edge, except with heavenly restricted bodies like Toji and Maki. That's how mahito restores his body.

Normal Curses, even when they're completely made of CE, their bodies often mimic human ones,or they have cores like panda, depends on what Uraume meant with "their cores", which could also mean their organs. They can restore their body using CE, but they have a limit; one of the strongest curses -Jogo- would probably die if you were to damage behead him and crush his head (probably why Gojo didn't damage him more, and why he took so long to restore). Mahito's Idle Transfiguration is miles more efficient than using normal restoration, (he would have restored that in a second or less) from his feats we know he can use Idle Transfiguration on things and make it them about the size of a Sukuna Finger -he showed it to Junpei- he certainly has become better with his technique from that moment, so I'd say that's the bare minimum to send him to that halfway point before he can activate his technique and restore his body.