I think there has been changes to the program. For instance, I was recently able to SDC on an international flight. When I called the DM line and asked to do it, they said that officially I can't SDC on international flights, but they could do it for me. I called about an upcoming flight next week to ask for a specific J seat that was showing blocked (not occupied) on ExpertFlyer, they unblocked the seat and gave it to me. They said normally I would have to see a gate agent but they could do it for me.
I think Delta is now identifying HVC's and allowing the DM line some discretion.