DDG launched in 2008, and according to the below, the deal between Yahoo and Microsoft didn't happen until 2009 at the earliest.
In July 2009, Microsoft and Yahoo! announced a deal in which Bing would power Yahoo! Search. All Yahoo! Search global customers and partners made the transition by early 2012. The deal was altered in 2015, meaning Yahoo! was only required to use Bing for a "majority" of searches.
Source: https://en.wikipedia.org/?curid=4325491