Here is the link to their exception/bulk fare policy:
https://www.delta.com/content/www/en...ion-fares.html Special fares are eligible for credit and MQM.
A diamond agent is not the one who gets to make this call. Call back. Ask to be transferred to a Skymiles Account Specialist. Its a seperate department. Or reach to Twitter, the Twitter agents will contact the correct department. You should get credit from this based on what you have posted. I would not give up.
The fact that the agent called the flight "marketed by Expedia" proves the point they did not know what they were talking about. Also...I have several Diamond friends that have reported lately that the Diamond line has just been going around wait queues but not actually being serviced by special diamond agents. One agent was apparently only on the second week of the job they said. So just because you call the Diamond (Or Gold or Platinum line) doesn't necessarily mean you are talking to anyone who is an authority.