My argument was meant to be that the tree may have to reduce the amount of fall wood it adds in order to repair and rebuild roots using it's limited amount of stored energy, if roots are reduced during a fall repot. I'm only speculating on that though
Yes, I've (so slowly) come to understand that. I will offer to you that in our tended pot culture it is not much, if any, effect. I've overdone root work in repotting that took an entire season for the trees to recover from. However, my instinct is that the roots recover in a couple of weeks of a properly done repot. I measured stem thicknesses of at least 8 trees of several species through the season. Week by week stem thicken more and more as spring progresses. Then shortly after the summer solstice this rate declines, week by week until cold dormancy sets in. Half of each species of tree was repotted in Aug/Sep and there was no significant disruption of the growth curve from the other trees that were repotted in spring. My repotting routine was to wash all the substrate out of the roots (done to weigh the young trees), invert the plastic nursery pots they were in, pruning the roots to the perimeter of the pot base, and then repotting the tree. The amount of root pruning is roughly equivalent to what I think to be normal practice with bonsai.
So, I don't think that what we do generally leads to the effects you were speculating, at least not to a significant degree.
I think everyone's concern about late summer or fall root pruning is that it may interfere with generating cold hardiness.
This story from Brent Walston of evergreengardenworks.com is the icon for this.
We know from scientific literature that most of the water and mineral adsorption capacity comes from root hairs that are the extension of the walls of individual cell walls a few millimeters behind a growing root tip (as such, root hairs are not visible to the naked eye). These cells die after a few weeks, but are continually being replaced by a growing root. Being superficial cells full of water, they obviously would be killed by freezing to any degree. Even freezing of the root tip, like air pruning, would kill the tip, but would also trigger a new tip to be created 'up stream' as long as that older, competent root tissue wasn't killed by freezing as well.
So why does it matter when roots are pruned in late summer?