This is supposed to be a general use tablet, so the use case should factor in how it is judged... A general tablet is not dedicated to pros manipulating photos all day long with the screen in perfect lighting at a perfect angle. A display should be adapted to the needs of the actual users and not the lab (just like all other specs).
According to the review, color accuracy, white point are basically very close to perfect (but not not the top amongst tablets).
Yet, is is funny how this part of the displaymate review is not here.
"iPad Air 2 matches or breaks new records in Tablet (and Smartphone) display performance for: the most accurate (pure logarithmic power-law) Intensity Scale and Gamma, most accurate Image Contrast, (by far) the Lowest Screen Reflectance, and the Highest Contrast Rating for Ambient Light."
So, basically, it is much easier to read stuff off it in any situation than competing tablet.
Apple focused on improving the contrast in every way possible. Legibility is difficult circumstance is key for a tablet.
So, again, as always, Apple balanced the engineering considerations which that person totally disregarded.