Indeed, no one talks about the privacy implications of referral links, although when it comes to the agreement with Bing, etc., I think everything is proxied. Not so much with the direct links to Amazon. Amazon knows when you are a DDG user.
Even though affiliate links are a low hanging fruit in regards to making money with search engines, it is not made transparent by DuckDuckGo.
They also use all the data they can get, they just tell us they get rid of the identifying information as fast as possible (IP address). I wouldn't be surprised if, from a privacy-standpoint, using Google and DDG as a non-logged in user is basically the same, especially if you use google with an ad blocker. The process of Google ads is basically to have a profile based on cookie data, which the user can easily control.