With that particular soft box that was his definition of the distance for his images to pop or what he feels his images pop. I disagree with the notion that the light source is not different coming from the panel or diagonal part of the soft box. Light coming from a center of a soft box is more DIRECTIONAL vs scattered light coming from the panel or diagonal side of a soft box.
Now Al, you didn't pay attention in Ohio. I had a brief discussion on the matter.
So let me go into more detail from a scientific perspective.
Distance between the light source and subject will define an image. The penumbra shadow, the part that receives some light, to the umbra, the part that receives no light are all effected by the distance and direction from a light source that will change it's characteristics from different distances and directions from a light source.
Shadows vary tremendously as a function of the lighting sources. They can be hard edged or soft edged and contain both an umbra and a penumbra area. The relative size of the umbra-penumbra is a function of the size and the shape of the light source and its DISTANCE and quality from the object. The umbra is that part of a shadow that is completely cut off from the light source, whereas the penumbra is an area that receives some light from the source. A penumbra surrounds an umbra and there is always a gradual change in intensity from a penumbra to an umbra.
A light source at large distances, in the simplest case, the objects produce umbra with sharp edges.
Distances of the light source that determine the quality of a shadow, enable us to conclude information concerning the nature of the light source. It's very important. Some photographers just take it for granted and say this works and fell into it by nature. Your happy go lucky photographer that hits and misses needs to pay attention to this law and start to understand it and pay attention to DETAIL.
I hope this gives you a better understanding of what he finds is the magical distance from his light source.