That's simply not true. Even as a technical user who knows what you're doing, you get a hard-coded choice of exactly five - Google, Bing, Yahoo, DDG or Ecosia. You can't add new ones at all. If you wanted to add a new search engine (like the one I'm working on, or any other), there is no way to do it.

But let's try it. Go to Safari > Preferences > Search and choose the "Search engine" dropdown, and you'll see you have a dropdown with only five options.

Google reportedly pays $15B per year for that top spot.

A work around is to add a safari extension like "Keyword Search" and associate a key word to your search. Its nothing like setting a default search engine for a normal user, but atleast there is a way for the slightly more technical user.

Though, I couldn't add a keyword to your site in firefox with its "Add a keyword" feature. Might be due to the javascript calling some url behind the screen.