The article says that the Kubelka-Munk model achieved color mixing that was "too realistic" and therefore difficult to use for the purpose of creating a distinctive brand palette. What this tells me is that they weren't looking for a model of how colors mix together, but a model of what colors look good together.
> "We know red and yellow should yield orange, or that red and blue should make purple--but there isn't any way to arrive at these colors no matter what color-space you use."
Well, that's clearly not true. In the HSL color wheel , orange is halfway between red and yellow, and purple is halfway between red and blue. Should have picked a better example.
Our problem is more abstract. We want to appeal to the user's notions of how pigments blend together, but in such a way that the operation is predictable and can be hand tuned to avoid unpleasant cases. Ultimately, the mixing behavior that we released is inspired by the physical world, but is tuned by our designers to produce what they consider pleasing results.
There's nothing wrong with any given color space. It was designed with particular goals in mind, and those goals generally did not include attractive blending through linear interpolation. So, yes, we developed a means of hand-tuning the interpolation such that it produces colors that look good together, which is what our users really cares about.