Inverse square law is only about the intensity of the light. 2x more distant is 1/4 as bright. It is NOT AT ALL just about point sources, its about softboxes and umbrellas too, IF you ignore the fabric and measure the distance to the actual flash tube. There will of course be dumb arguments, but inverse square law is NOT measured from the fabric surface, the fabric is not the light source. Only math students studying calculus like to consider the flat panel. :) Photographers have different concerns. And we generally more conveniently use light meters instead of a measuring tape.
Soft light is about the largeness of the light diameter. That is much more than just diffusion. We can use a large light, or placing it close makes it appear larger, as seen by the subject, so soft is about both close and large. It is pretty hard to get too much of either large or close.
A light at a distance equal to half of its size: seen as 90 degrees width
A light at same distance as its size: 53 degrees width (this will be quite soft light)
A light at 2x the distance as its size: 28 degrees width
A light at 5x the distance as its size: 11 degrees width
An 8 inch softbox at 6 feet (9x): about 6 degree width
A 2 inch flash head at 9 feet (50x): about 1 degree width
Our Sun (865,000 miles diameter, 107x): 0.5 degree size.
The trigonometry: degrees of angle = 2 arc tan(radius/distance)
So, a one foot diameter light size placed 10 feet from the subject is seen by the subject as being 2 arc tan (0.5/10) = 5.7 degrees wide, or from 2.8 degrees either side.
But a four foot light placed 2 feet from subject is seen as 2 arc tan(2/2) = 90 degrees wide. So this light is also coming from 45 degrees either side of subject, above and below too, and so all those angular light rays are self-filling the shadows made by all the other paths, and are even wrapping around behind the subject in some degree. This is why "large" and "close" is the definition of soft light.
So just putting a diffuser on the small light is NOT at all the same thing as soft. Putting a diffuser on small light just scatters the light outward so that most of it misses the subject entirely. It has no diameter to angle any side light back towards the subject, to fill shadows made by other light paths.
So yes, inverse square law does affect the degree of light fall off behind and in front of subject. Greater distance does give less fall off. But greater distance is the absolute pits for softness.
If using tiny lights (bare flashes), softness can't matter, and 6 to 10 feet could have the fall off advantage you mention. A room may need a lot of depth, but a human portrait only needs about one foot, no big deal. Focus depth of field is surely more important.
But large lights at 6 to 10 feet is poor advice for portraits. The best rule for softboxes and umbrellas for portraits is "as close as possible" (meaning just far enough to keep them out of the camera view, barely). I'd say 4 or 5 feet ought to be easily possible for umbrella fabric, and 2 or 3 feet for softboxes.
This makes these large lights appear huge... and soft.
Same size light as its distance is a good rule of thumb to provide decent softness. Four foot light at four feet works fine.
The main light is placed maybe 45 degrees high and wide, to intentionally make modeling shadows. These shadows can be pretty dark though.
So we use a fill light, which has to be frontal, to weakly illuminate and partially fill the shadows that the camera lens sees (and not make any more shadows). Lighting ratio is the ratio of these two light strengths (visibility of shadows remaining, some degree is desirable). But the fill light is frontal, and so has to be back near the camera so the camera can see around it. Above the camera is a good place for it. But it is just filling a few dark shadows, and because frontal is specifically NOT making any new shadows, soft is NOT its concern (no shadows to soften).
Overkilling this point, the fill light is close as possible to the lens axis, lighting exactly what the lens sees, so it makes no visible shadows needing fill. It merely lightens the existing main shadows that the lens sees.
The main light does need to be large and close and soft.