I don’t have any Apple hardware, but my recollection from seeing those who do a few years back (including when they first introduced the Retina displays) is that all of these laptops were defaulting to non-integer scales, and at least a little uncomfortable at 2×.

Except that they haven't been setting their displays to integer scale for a few generations now.

Indeed. On an external screen that would be most comfortable at about 125% MacOS will only offer you 100%.

They used to be, but not anymore. However, they switched, because even if not pixel perfect, it is good enough. At sizes used by their laptop, most users are not going to notice that it is not pixel perfect.

The system they use gets them something: 1) apps do not have bother with fractional scaling, so they do not get to bother about rounding errors and 2) they get to use output encoder for scaling, which is essentially for free, instead of GPU, which will take its toll on power consumption and GPU load.

This has nothing to do with device resolutions. This is about does one want UI elements to be scaled anywhere between 1x and 2x.