Yes, field of view is what really matters, but scaling at the OS level (like TFA's author is doing) is a good way to correct for unusual fields of view. Got a higher-than-typical DPI and/or ridiculously far away screen and/or vision impairments? Scale the OS up. Got a screen with lower-than-typical DPI and/or touching your nose and/or amazing vision? Scale the OS down. You need the entire OS scaled, not just web pages.

"CSS pixels" are kind of a roundabout way to conceptualize it, but they solve the problem of proper readability once the OS scale is correct for the user. The combination acts as a proxy for field of view.

Then there's detection of course/fine pointing devices, as already discussed, to further augment the layout for proper target area.

I don't believe anything else will matter if the above is accounted for.