The airline cannot be expected to sit with any child that books a ticket to make sure no one touches her. They aren't a parent, they're a transportation company.
Delta requires children under age 14 traveling alone to enroll in their Unaccompanied Minor program, under which Delta takes responsibility for their care. They act in loco parentis in this case, much like a school. They sign for "custody" from the adult dropping off the child at the origin, and have "chain of custody" paperwork throughout the trip, until the child is signed back over to the receiving adult at the end. They are indeed responsible for the child's safety in this case.
I think they are responsible for making sure the child gets where she needs to be so she doesn’t get lost in plane changes and things.
I just don’t see how they can be expected to know that something like this report indicates is happening if the girl doesn’t report it and the other surrounding passengers didn’t appear to notice. The flight attendants have a whole plane to serve. They can’t help her if she doesn’t alert them.
I think the very fact that we don’t know who the man is, is suspect. They know who was on that plane. If there was any basis for the complaint, they’d have been able to get a name.