That doesnt say I'm incorrect. the official pr says that they combine over 100 sources including bing without specifying how heavily they rely on any single source. But I've seen many times comments from ddg staff on here saying its mostly bing.

Yahoo hasn't had a search back-end since about the Bing deal. Most everyone on those teams was laid off while they transitioned over. Yahoo BOSS is backed by Bing.

src: http://searchengineland.com/yahoo-boss-moves-from-yahoos-ind...

src2: I worked on the Yahoo Search team at the time of the deal.