Sites on tennis rackets are 100% valid results when searching for the term "racket". DDG isn't doing anything wrong there. The only reason you're getting the results for Racket the programming language only, as you wanted, from Google is based on the information they have on you and your past search habits.
DDG explicitly places an emphasis on privacy and non tracking. You cannot expect the same targeted, contextual results that Google gives you if you don't want to give up some degree of privacy. People who do not want to give up that level of privacy and use DDG do so with the understanding that they lose that level of personalization.
You may be happy to provide personal information (browsing habits, clicks, etc) in exchange for better results. If so, then Google is the search engine to use. Some people do not accept that tradeoff; this is the market that DDG is for.
I'm pretty sure they meant Racket-specific queries that have enough context to make clear it's the language, not just plugging the term "racket" into the search engine. The point being that Google is better at picking up the context of queries, even when they don't have your search history.
DDG explicitly places an emphasis on privacy and non tracking. You cannot expect the same targeted, contextual results that Google gives you if you don't want to give up some degree of privacy. People who do not want to give up that level of privacy and use DDG do so with the understanding that they lose that level of personalization.
You may be happy to provide personal information (browsing habits, clicks, etc) in exchange for better results. If so, then Google is the search engine to use. Some people do not accept that tradeoff; this is the market that DDG is for.