I think most DDG fans would be thoroughly surprised DDG ever popped the hood or tried to verify their privacy claims, which is why I don't think DDG ever will.

Better to elude to a nebulous definition of privacy rather than give specifics. Even as a privately traded, for-profit company based in the U.S. with zero accountability for its claims, DDG doesn't even have that much IP to lose (as a Bing reskin) should it ever fold under a class action lawsuit.