Don't they IDB the last person who checked in (whether or not they have a seat assignment) vs. the person without a seat assignment? That was my impression, but it might be wrong.
There are three categories of people with respect to IDB. The first is Medallion members and those in a premium cabin; they will be accommodated first. The second is those with boarding passes; they will be accommodated second by class of service and then checkin time. The third group is passengers without boarding passes (ie a seat request card); with some secondary priority for IROPS, FO, ST E/E+ and then sorted again by class of service and checkin time.
Full details are on page 42 and 43 of the COC, located here:
http://www.delta.com/content/dam/del...rriage_dom.pdf