Welp, I dug further and found that many of the site maps closely resemble a lot of other locations in our solar system but not quite. I also noticed some patterns between several of the site maps -- so I think
their artists used real planetary maps from our solar system and used parts of them to construct 'original' site maps... at least for a lot of them.
Many are taken from Mars, Callisto, Mercury, the aforementioned asteroid and several other moons around Jupiter and Saturn.
I'm not that talented (loosely used) so I've just been grabbing planet maps, finding interesting looking parts, scaling them and cropping them.
It looks like they reduced the color count to 4 and then just set their palettes to determine the height maps (there are many maps where flat surfaces are marked as difficult or impassable when it should be the other way around). So I'll continue doing this by hand as I've been doing -- using the four shades of grey in multiples of 50 to determine height mapping, blending, reducing to 4 colors and then hand touching afterward.
Kinda fun to see that I developed a similar technique to what they did way back when.