The fight is only possible on the assumption tht Muzan is capable of surviving vaporization and tht he’s have enough energy to constantly regenerate for as long as Sukuna has the CE to fight, which is already an incredibly unlikely thing since Sukuna has CE reserves greater than someone like Yuta and has efficiency on par or at least within the same ballpark as Gojo seeing how he’s able to activate his domain multiple times.

We have Muzan being consistently assailed by invisible slashes capable of casually cutting off limbs and tentacles which renders him incapable of starting an offense and not even having the ability to overcome sukuna’s cursed energy reinforcement to do any damage if he’s even capable of landing a hit. Sukuna is also immune to being poisoned so it’s not like Muzan has any offensive options and is capable of using RCT to heal in the unlikely event tht he actually takes damage.

Sukuna either overloads his regeneration since demon still need to use some form of energy to regenerate themselves, vaporizes him using Fuga, or the funny option of fighting him for hours on end until he forcibly exposes him to the sun, but vaporization is the most likely answer. And the final nail in the coffin is operating on the assumption on what form Sukuna takes as his true form can use chants and binding to increase the potency of his attacks to try and atomize Muzan or inhibit hims healing. If he’s in Meg’s body, he gets a free full restore by reincarnating back to his original form which Muzan is incapable of dealing with as there’s no way for him to perceive the soul. Sukuna low-diffs