I am sure there is a presentation bias. I am sure that there are plenty of cases where a user cannot tell if a set of results are good or bad for a specific query. I am sure some users have trouble communicating why they think a specific result is good or bad.
However can people tell the general quality of a search engine? Absolutely. Google's simple aesthetics certainly helped them, but they didn't become the top search engine by luck and aesthetics alone. AltaVista, Yahoo, Lycos, Excite were all junk compared to Google, even the first version. It was very obvious.
The research I'm citing is from the late 00's. At this point in the game, differences in search relevance are in the measurement noise.
2. DDG has contact info. I maintain that it's better to send data directly to them if one is troubleshooting it or helping them improve the product. Hoping a random employee is on same tech forum is a stretch despite it happening here more often than average. So, if we all did search data, we'd post it on a dedicated forum they host after we email them about the project.
2) There is at least one ddg developer here (not me), and HN is generally a good source of constructive feedback for startups. Maybe the original commenter's query string would help ddg or a competitor improve their product.