Maybe it's true that as the downhill approaches 11%, speed at a given rate of oxygen consumption COULD approach infinity, less other unaccounted for elements such as wind resistance. At such a steep decline, the runner's would no longer be limited by oxygen consumption, and would be able to run as fast as his/her legs could move. Therefore, the downhill equation would need to approach some constant which is the maximum attainable speed for the runner.
However, at such speeds, wind resistance and resistance from the friction of the road, shoes, etc, would contribute to oxygen requirements as well as the maximum attainable downhill speed.
I know nothing about physics, but I'm guessing gravity would also be in the equation somewhere.