I would argue that behavior is idiomatic for macOS but not idiomatic for web browsers. Keyboard navigation of all elements has never been the default in macOS. Tab moves between input fields, but without turning on other settings, almost never moved between other elements because macOS was a mouse first OS from its earliest days. Web browsers often broke this convention, but Safari has from day one not used tab for full keyboard navigation by default.

And this highlights something that I think the author glosses over a little but is part of why idioms break for a lot of web applications. A lot of the keyboard commands we're used to issue commands to the OS and so their idioms are generally defined by the idioms of the OS. A web application, by nature of being an application within an application, has to try to intercept or override those commands. It's the same problem that linux (and windows) face with key commands shared by their terminals and their GUIs. Is "ctrl-c" copy or interrupt? Depends on what has focus right now, and both are "idiomatic" for their particular environments. macOS neatly sidesteps this for terminals because "ctrl-c" was never used for copy, it was always "cmd-c".

Incidentally, what you're looking for in Safari is either "Press Tab to highlight each item on a webpage" setting in the Advanced settings tab. By default with that off, you would use "opt-Tab" to navigate to all elements.

System Settings -> Keyboard -> and toggle Keyboard navigation.

I'm not sure why this isn't the default, but this allows for UI navigation via keyboard on macOS, including Safari.