A wheelchair return time is not the same as DAS. If the wheelchair resolves the guest's need(s), then a DAS will not be issued.
The wheelchair return times are given when the parks are quite busy at rides with separate wheelchair entrances -- often rides that would get very backed up for those in wheelchairs resulting in an extra long wait. My understanding is it seems to be working quite well. Wheelchair return times may or may not be in use at any given ride or on any given day, it depends how smoothly things are going at the time.