This has nothing to do with search engines, it substitutes the argument from the search bar into the destination url. "wp XYZ" -> "https://en.wikipedia.org/wiki/XYZ"
It has everything to do the browser feature known as "custom search engines", which is a URL replacement capability that was never limited to actual search engines.
As I said before, Mozilla is going to merge your bookmark keywords with custom search engines: https://bugzilla.mozilla.org/show_bug.cgi?id=1650874