I'm aware that 'pixel perfect' doesn't strictly mean 'totally fixed layout'. That's not really relevant here.

You can't have, for instance, consistent alignments down to the pixel level, on systems with arbitrary DPI emulation, e.g., when you set DPI manually to something other than 96 in Xorg. This is where the usual caveats for 'fractional scaling' come in (because you have to render at a higher resolution and then scale it down), why such scaling often results in blurry text, why HiDPI displays are required for the normal scaling on macOS, etc. Such changes come in part in pursuit of enabling 'pixel perfect design' on the desktop, and they suck.

Commenters on this site will frequently point out things that are apparently misaligned by a single pixel. What is their complaint, if not that the UI element in question fails to be pixel perfect?

Whether the System Preferences pane must have a fixed size in order to achieve 'pixel perfection' is perhaps debatable. That such inflexibility makes pixel alignment easier is obvious. You think Apple have some other, better reason for System Settings being completely unresizable? What is it?

The point was that unless an engineer has practiced implementing designs exactly, they often miss the details and don’t _see_ the details they’ve missed.

I say this as someone who used to not see the details, and then worked at a company where it was expected that we implement designs exactly. Ever since then I can’t _not_ see UI design details.

It’s like shopping for a car, once you’re biased towards a specific model you see it everywhere.