They have done the opposite as of today, the changed their BOSS API, the main data source of Duck Duck Go, from free to $1.80 per 1000 queries: https://developer.yahoo.com/boss/search/ (There is a section "Who's using Yahoo BOSS" -> Duck Duck Go, and a "Pricing" section below)

What would they do with it? It's doesn't have very many users. It relies mostly off of Bing. The results aren't as good as even Yahoo's or Bing's.

So they acquire it and promote it as this big privacy alternative to Google search seperate from Yahoo search (which is tied to obligations of their deal with Bing). Assuming that all search engines they make aren't subject to the same obligations with Bing, they get a nitch brand and a opportunity to build a new search engine from the ground up? Oh and for all that work they get no money from targeted ads and user data?