... or other sketchy freeware?
The site you link to says as much, that some bullshit freeware and shareware downloads will change your search engine preferences, and aggressively keep them changed.
--snip--
I havent installed DuckDuckGo. How did it appear on my browser?
We have already mentioned that browser hijacker spreads bundled with free programs that users download from the Internet. The developers of PUPs are aware of the fact that computer users are lazy, brave and inattentive. They always choose Quick or Recommended software installation mode and just several times click Next and Agree buttons.
They do not realise that by using this setup and clicking Agree they let PUPs in. To avoid DuckDuckGo hijack or other suspicious add-ons, you have to choose Advanced or Custom setup. Then you will be able to see the list of additional programs and decline their installation.
Besides, you should always download software from reliable sources and read all documents provided by the developers. The Privacy Policy, EULA or Terms and Agreements might not be the most interesting pieces to read, but you have to secure your computer and your identity.
--snip--
https://novirus.uk/duckduckgo/
There are probably a lot of sketchy or dangerous sites that don't want you using google as your default search engine because google flags them as sketchy or dangerous sites.
The DuckDuckGo search engine itself is legitimate according to Apple, Microsoft, Yahoo!, Wikipedia, and other business partners.