How sure are we that Sukuna actually melted the block of ice here? He doesn't have any passive heat abilities as far as I know, there's no sign of his flames in action here, and Kashimo describes it all as just "theatrics" for delivering the item inside the ice to Sukuna. What if Sukuna snapping his fingers was just a signal, or Uraume just released their ability on their own? We know Uraume's ice can melt instantly when they released their technique like in chapter 136.
EDIT: Now that I reread it, Sukuna didn't even snap his fingers. He just raised his arm.
EDIT: Now that I reread it, Sukuna didn't even snap his fingers. He just raised his arm.