The use of metaphors worked when people didn't know what a computer was, and the only way to make its UI make sense was to mimic real-world objects. Now, this is no longer necessary. It's been 40 years.
While I agree that new macOS icons aren't great (see the Game Center icon), the old ones were silly. I'm 35 and I have probably seen an actual contact book only once when I was little. It doesn't make sense to have a skeuomorphic contact book as an icon for Contacts. The old icon for Pages? I don't even know what that is, I'm not into calligraphy.
The only thing I'd agree with is hiding UI controls. Apple has been making its apps less usable to make them look pretty, hiding important elements in an attempt to declutter the interface. I hate how Safari hides the full address in the address bar, for instance. Or, how they removed scrollbars and force people to actually scroll every piece of the interface to check if there's something more to see, while before you could tell just by looking at scrollbars. Of course, there are settings to go back to the old behavior for both my examples, so power users are fine, but I fail to see how these moves improve things for regular users.
I also disagree that Steve Job's death was detrimental to macOS's UI. He was the one who kept Apple looking outdated with his obsession for skeuomorphism, I'm glad they went for a flatter look right after his death.
Of course, everyone's taste is different, but I still think this is a bad article.