The simple matter is Numbnuts didn't look before he operated the switch;
Easy , this is investigation done like 30 years ago ' the operator has srewed up . no need to change anything ".
I do not know how many millions flight hours the 737 has,and how many tens of thousands crews have flown it since 9/11, and this appears to be a one-off incident. The question is :can it happen again with a different "Numbnuts" ?
If the answer is no, then send the numbnuts to re-education .
If the answer is yes, then change something.(i.e relocate the door unlock knob elsewhere , preferably in a standard location )
End of the story.