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.
0: https://forums.macrumors.com/threads/10-8-new-wallpaper-back...