It has only 64 shades (6-bit) instead of your more typical 8-bit, so this more limited colors than you'd expect.

Banding is more typically seen in monochromatic gradients - black and white or shades of sky blue. But this image is a yellow/blue gradient, and it's super unnatural to have the RGB channels transition at the same time - typically they transition at different locations and that helps smooth things out.

I agree, almost looks like it’s a CGI image generated specifically to illustrate banding.