Right. Sulfured fuel was before my time, so I can only guess why people concluded ULSD solved the buildup problem. If the crud used to smell like sulfur, i can see why they'd reach that conclusion. If not, then i dont know what.
I'm more curious about the old fuel giving higher mpg, than the buildup question. If the sulfur content was indeed beneficial, then it should be an extremely cheap and easy fuel additive -- rather than paying $70/gal (55cents/oz) for some name brand snake oil. If the snake oil actually delivers results (at 4oz snakeoil per gallon of fuel, i.e. 14 cents/gallon of diesel) then a 10% boost would be break-even at $1.40/gal diesel cost. But part of the psychology of MPG improvement is, when you're consciously trying to improve mpg, you will drive more conservatively. So when guys report Snakeoil Fuel Treatment(tm) got them more mpg... well, maybe the snakeoil did it, or maybe they changed their driving habits a little bit.
I could also swear that diesel is a good mpg booster for unleaded gas. I used to see a 10% boost when I topped up my Honda on Friday afternoons from gas cans. Those same cans were used to transfer diesel into equipment during the week, so there were a couple ounces of residual D2 when the cans were filled with unleaded. I don't drive gassers anymore, so it's not something I feel like exploring. But I'm certain the improvement wasn't psychosomatic, since the gas was free, and it was a 2-3 hour drive at the end of a 60 hour week that I just wanted to be over.