Comparing to JPEG seems somewhat dishonest (or thoughtless), considering the large parts of one color in some of the images. PNG and other algorithms not designed for detailed pictures would probably do better.
It's not representative of the performance of JPEG becasue You typically don't encode JPEGs at that bit rate.
It's basically taking advantage of the fact that JPEG dips first. http://article.sapub.org/image/10.5923.j.ajsp.20120205.04_01...
Also be wary of things that report compression rates rather than bits-per-pixel. You can get the same image quality at 200:1 if you start with 48 bit colour, The bits per pixel would be the same but if your original data is excessively precise you can throw a lot of it away at no perceptible cost.
JPEG is the appropriate comparison, as the authors are solving the same problem as JPEG tries to solve (i.e. a good enough lossy compression for natural images).
You're right that PNG does well on pictures with only few colors like diagrams but who knows how the edge-detecting PDE format would do, it might compress just as well with the same quality.
So I look at PDE and notice that its few points and colour contours bear a lot of similarity to a basic vector format. I wonder if a simplified vector format that utilises tracing combined with blurring effects would best PDE.
For a vector to encode a sky gradient you would have to have a shape and a specific falloff that blends into another shape and falloff to create the gradient.
The big idea is to describe the image intensities as a continuous surface which can be approximated in 3+ dimensions, not a patchwork of vector areas.