As the article shows, the close button on MacOS classic was basically an empty box, but on mousing down on that box, it transformed into something that looks a bit like an x. I'm basing this on what I can see from using [1], but from my possibly inaccurate recollection of using the real thing in the 80s and 90s, some versions of MacOS had an even more "x like" mouse down image on the close button.