The claim in that phrase of the article is that a "unit test" was originally supposed to mean "a test that runs unitarily, by itself". Conversely, the more common interpretation of the term is "a test for a single unit of code".

So, according to that phrase, units are not part of the system. The units are supposed to be the tests.

Note that I don't agree with this semantics game. But that is the intended meaning of that phrase.