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