nobody has pointed out that there are treadmills on the market (such as elliptigo) that are specifically designed to allow you go to faster with less effort.
Those may or may not be useful, but they are used by some elite training programs. Some pros have run some very fast times using them as part of a training program. Of course, obviously some pros have also run very fast times while not using them as part of a training plan program.
But my point is, there is nothing wrong with setting the treadmill at 0%. I've always thought it was a bit silly for everybody to set it as 2% to keep it from being "too easy." A treadmill is never any serious runners first choice for a workout, it's always because it allows for some sort of scientific testing (vo2 testing, for example) or because there is some reason they can't run outside (blizzard conditions, crazy heatwave, etc.) So to me, insisting on setting it at 1 or 2% incline seems a bit insignificant.
So to answer the OP'S, question, the idea you posed is similar to the marketing reason for the elliptigo treadmills. The idea is that they allow you to take as much as 50% off of your apparent weight, to in theory allow you to run at race pace with a much reduced effort.
I would say though, I would find it very difficult to run at 10k pace for 20 minutes on a treadmill even at 0%. I wouldn't feel that this would be a tempo effort. Maybe somehow you take to the treadmill better than I do, though.