Under the circumstances, it sounds like you got lucky. While the DL Amex Platinum card has sometimes offered 15K MQM (and 40K RDM), the DL Amex Gold has not. So, most likely someone at Amex or DL messed up to your benefit.
Although unlikely, I suppose another possibility is that someone with a DL Amex Reserve card hit their $30K or $60K spend requirement and accidently had their 15K MQM bonus deposited into your account (e.g., by inputting the wrong SkyMiles number... a fat finger error).