Coach,
I see what you're saying. I think we generally agree it's perhaps just the percentages of effort coming from back versus legs we are differing on, not a big deal though. The posterior chain works together and the amount of effort from the different muscles will depend alot on the lifters existing strengths.
Yeah sumo squats tend to be hit or miss depending on your body type, I know some people have said they also do not like the inwards forces it can place on the knees.