My interpretation is that the following happened:
- Sukuna made Megumi's soul summon Mahoraga's wheel
- During the 5 Domain struggles inside the domain barrier, Mahoraga was adapting to Unlimited Void
- Then we saw the wheel turn, but didn't know that the wheel was over Megumi's head the whole time
- Then Mahoraga got summoned inside Unlimited Void and destroyed it because it had adapted
- Problem is, Sukuna used Megumi as the medium for the adaptation process, so now if Sukuna summons Mahoraga himself, he would need to go through the adaptation process again. And that would give Gojo a chance to one shot it.
But now neither can use Domains, so we'll probably see more hand to hand combat and basic techniques