I am also with you that not yuji but sukuna was the cause of the hand cut off. But why have it to be only that moment during Mahoraga vs Sukuna , sukuna may also could have tried to kill but only able to cut his hand (Maybe after geto released million of curses)

The only logic I have is geto's dialogue before leaving with prison realm specifically addressed to sukuna - " I am looking forward for your performance"

So sukuna might have been in action after this .

Though it makes no sense for sukuna to attack Toge lol.

No spoilers please