That image has too much noise. You need smooth gradients to get banding. The pony, the smoothest part of the image, has a lot of sensor noise (more chroma than luma but still) in the 3-6 pixel frequency range.
I was trying to prove my photo point ("If you were to repeat the same comparison with a realistic photo, the differences between the original and the jpegmini version, there are some, but you have to look hard, would be even less noticeable.")
That image has too much noise. You need smooth gradients to get banding. The pony, the smoothest part of the image, has a lot of sensor noise (more chroma than luma but still) in the 3-6 pixel frequency range.