I have to disagree. The amount of fill flash that is needed is not determined by any camera setting. Rather, it is a matter of the strength of the ambient light. Since by definition you are trying to "fill" the shadows of your subject, doing so depends on the amount of illumination of the non-shadowed part.
What is true is that you tend to need a low ISO for fill flash because your camera's sync speed (non-auto FP) limits how fast your shutter speed can be. "Sunny 16" says that at ISO 100 and 1/250, you need an aperture of about f/11 in bright sunlight. At ISO 200, it's more like f/16.
So, yes, you may want to shoot ISO 100 for fill flash, but it's not to give your flash greater distance. And for regular, non-fill, flash, it is a higher ISO that gives greater flash range.