With just two colours it's hard to see the intermediate values because our eyes tend to blur together colours; is that an intermediate value or sharp edge that I'm seeing fuzzily due to poor focus? With an intermediate tone, such issues are pushed down to lower resolutions - with yellow-red-blue I know for sure that if it's red it's an intermediate, but is the orange area a mix of lots of small yellow and red domains or is it a field of intermediate domains?
Rinse and repeat.
The article seems to say "this scale is better" whilst also describing how other scales can be better. The evidence they present says to me "choose scales according to the data, the desired use, and the observer", of you ship the data used to cover a visualisation along with that visualisation then people can use their own scale altered to the purposes (and disabilities) they have.
Any scale that only uses hue is terrible, no matter how many intermediate steps. Scales that only accidentally utilize luminance (due to human vision being most sensitive to green, then red, and by far the least sensitive to blue) are only marginally better. Indeed, the rainbow scale is atrocious also due to the perceptually most luminous color being somewhere in the middle of the range, so people with red-green deficiency cannot even easily use luminance cues to make sense of the data. Not even mentioning the fact that human vision is simply much better at distinguishing luminance differences than hue differences.
Rinse and repeat.
The article seems to say "this scale is better" whilst also describing how other scales can be better. The evidence they present says to me "choose scales according to the data, the desired use, and the observer", of you ship the data used to cover a visualisation along with that visualisation then people can use their own scale altered to the purposes (and disabilities) they have.