It wouldn't surprise me if DL had some sort of program that trains people in all departments to jump in when things like this happen. When I used to work for a major electric utility nearly everyone in the company had "storm duty" assignments including high-ranking fresh out of biz-school management folks. Everyone had to get re-certified each year and when major storms hit most other functions stopped. We would even loan out people to other utilities for bad storms. I would assume airlines do something very similar.

I actually expect this sort of mobilization is somewhat common (a few times a year) related to weather events. I don't think it would make sense for DL to carry enough dedicated reservation staff to cover the occasional massive surges that occur as a normal part of the airline business. I also to hear all to often of multi-hour waits like you do over on the UA boards.