macOS renders content on my 100 PPI monitor at exactly 100 DPI; 1:1, no scaling, so everything looks crisp at the pixel level. The scaling only happens on high-DPI displays (I think the cutoff is around 150-200), and for me at least, ~250 PPI is more than dense enough to not see any individual pixels and thus no aliasing artifacts. Since you like pixel-perfect rendering even at very high resolutions, perhaps you have superhuman vision. My eyes are decidedly average. :-)
>I hardly ever use windows spanning multiple displays
Me neither. My issue is that the windows are rendered at different sizes even when they're not spanning both displays: if I dragged the window in the example photo upwards to sit entirely on the top display, it would stay huge, whereas if I dragged it downwards to sit entirely on the bottom display, it would stay small.