My information does come from historical sources, although I'd be hesitant to conclude it doesn't apply beyond them. In the sources you cite, they detail for example how Apple moved on from defining a reference constant to just speaking in terms of logical-to-physical pixel mappings. This took a while for me to wrap my head around, since I considered that basically nonsensical, but it means them having taken exclusive ownership of the reference PPI, preventing third parties from strongly relying on it or expecting physically precise sizes. This doesn't necessarily mean such a constant (or constants per device class) do not exist, it just means it's under wraps. 1x has to mean something in terms of density, otherwise apparent sizes of screen elements wouldn't be transferable across devices. Unless they really are just eyeballing it or just "letting jesus take the wheel", which would be entertaining, but I find that unlikely.
> Per [3], it seems that non-Retina displays should be 100-120 ppi and Retina should be 200-230.
Maybe. I'd be cautious to just accept it, some person writing that on their blog is not any more credible than me writing anything here as a comment. Random bits of Apple documentation still references 72 PPI as the 1x reference point for example. [0] [1]
It's possible though that in the spirit of the aforementioned, they did "secretly" move to 110 PPI or something as their reference constant, and so by that 2x is indeed 220, giving GP what they see, but that just rubs me so strange. But I mean, if when you open the display settings on a Pro Display XDR or similar, and it says the scaling factor is 2x, and you can reproduce this kind of scaling selection across a wide array of devices all hinting at 110 PPI being 1x, then there we go, apparently their current reference PPI then is 110.
[0] https://developer.apple.com/design/human-interface-guideline...
> Point size based on image resolution of 72 ppi for @1x and 144 ppi for @2x designs.
> Point size based on image resolution of 144 ppi for @2x and 216 ppi for @3x designs.
[1] https://developer.apple.com/documentation/appkit/nstouch/dev...
> The range of the touch device in points, such as 72 ppi.
(admittedly a pretty random source to cite this last one, doesn't explicitly support anything)