I got this as well and am a little surprised.
Not sure if this is reverse engineer-able. It's probably a combination of things, but here are my DPs:
- I've never had status with Delta or any Delta credit cards.
- I do have the Amex Platinum and Gold cards.
- I flew one cash fare on Delta One last year.
- I have two one-way domestic flights booked, but zero logged MQDs.