You got it.
I just fixed a couple of long-unnoticed bugs in this area as well: the diagrams only reorder properly on the first document window. If you do a Window/New Window and then reorder the diagrams, only the original window will reorder (regardless of which document window you ran the command from). The 'SaveDiagramPositions' property will also disappear if you change the extensions (with the extension manager) when two document windows open, then closed one of the windows. These bugs will be fixed in the next public release, but given that no one has reported them before I'm not treating them as critical.