Why is deltaE not used as a measure of error?
Delta E is used extensively in vision research, however the study you sited only tested hue recall (all samples and test responses were designed to vary in hue and minimally in luminance and saturation). 

This is one possible criticism of the studies. One of the major improvements in Delta E 2000 is a hue dependent rotation. We are more or less sensitive to certain hue regions. While this is one source of criticism, they are generally relying on large color differences to differentiate successful trials vs. unsuccessful trials and Delta E does not have any meaning for large color differences. 

A more accurate reinterpretation might be to say that they aren't calculating color differences at all but rather the variation of the response distribution and the underlying probability density functions. The variation analysis is not dependent on individual color difference calculations.