Ironically previous versions of macOS also used brushed metal heavily

That's the reference. Brushed metal was a controversial design feature of early Mac OS X, unevenly applied across the built-in applications. (It was kind of supposed to be for media browsers, but then the Finder and Calendar and many others were also brushed metal for some reason.)

It debuted already in 1999 in QuickTime Player, years before Mac OS X:

http://hallofshame.gp.co.at/qtime.htm

The rumor was that Steve Jobs liked this style a lot, so it spread to the operating system despite the widespread external criticism (like the link above).