For me, it looks like the PDE format is well suited for photographic images with abrupt color changes like the Svalbard picture. The sky is a gradient, it is no

For me, it looks like the PDE format is well suited for photographic images with abrupt color changes like the Svalbard picture. The sky is a gradient, it is not just one color, PNG would compress very bad on this one. 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.