Are ShortPixel images progressive or normal?

ShortPixel will automatically generate "progressive"  (interlaced) or "normal" images depending on which version will result in a smaller weight. As a general rule, very small images will be normal, while larger images will always be progressive after optimization, as they are smaller this way.

In any case, don't worry about this: ShortPixel will always make sure the resulting image is the smallest possible one.