Speculation time?
I think Sukuna smiled cuz he sensed Megumi. A few interesting things here:
1) Megumiโs technique when fully mastered is supposedly on par with (at least some of) the Gojo clanโs techniques.
2) ALSO, because Sukuna exorcised Mahoraga, that means itโs power should have gone into Megumiโs summons right?
3) Megumi with a Mahoraga+ level summon vs Yuta and Rika?