This is mentioned nowhere in the privacy policy, and it's not at all clear to me that the things that are mentioned on that page are not identifiable. The requests being made to improving.duckduckgo.com, according to the browser console, contain things like browser, OS, language preferences, etc. Take a look at this: https://duckduckgo.com/util/u598.js.

It creates an incentive to track users (e.g. to find out which users pass the A test).

Also, in this case there is no way to opt out.