It's not mentioned on the page, but at least 10.0 to 10.8 are AI upscaled -- you can tell from the artifacts when looking at the image closely.
I don't think it's reasonable to call this an "archive", as the link does, when the images have undisclosed changes to them and are clearly not the original files :/
Thank you for saying it, undisclosed "enhancements" like this just remind me of the botched Ecce Homo painting restoration. A request to the world: If you're going to post an "enhanced" version of something, at least share the original too.
Hmm, this forum post[0] is from 2012 and includes a high res photo from the OS, but it seems nearly visually identical to the photo on 512pixels.net. I don't see almost any artifacts that give me the impression of AI upscaling.
That 10.8 wallpaper you linked looks original. 3200 x 2000 appears to be the original resolution for Lion and Mountain Lion (also note how the version on this site is cropped).
10.8 is less obvious so hard to say with 100% certainty, but if you look at, say 10.6 it's super obvious.
If you look at the bright star at the top left, in the original it has a clean lens flare. In the version from this site, the diagonal flare has a zebra stripe pattern. This is characteristic of AI upscaling of a very thin diagonal line.
I don't think it's reasonable to call this an "archive", as the link does, when the images have undisclosed changes to them and are clearly not the original files :/