There is certainly also a device limitation. I would expect that with less than full 24 bits of color, some fields might just look the same and the results do not depend on your vision any more.
Let's say the device has a "24 bit color display".
What about eye protection color shifting? This limits the color space used could reduce the effective remaining bit depth.
Or maybe they do temporal dithering to get more bit depth? Or maybe the 24 bits are already achieved with temporal dithering?
It does not need to be a calibrated display, but a cheap tablet in sunlight will be worse than a color grading monitor in a reference environment.
I hope they also register the devices used and analyze the statistics on that.
Calibrating a monitor is intended to ensure that colors on your monitor closely match colors on an ideal reference monitor. That's not the same thing as ensuring that two different colors on the same monitor actually show up differently; that's a much looser quality standard, because even a badly mis-calibrated monitor may still show both colors as distinct wrong colors.
I would only expect poor calibration to break this test for colors near the edge of the display's gamut, or if there's a drastic-enough shift that the color space's lack of perceptual uniformity means a numerical difference that should have been visible ends up in a different part of the color space where that same numerical difference is not perceptible.
Well, I initially ran the test on my cheap vivo phone when I switched to my Motorola the difference was very noticeable, there's obvious color crushing/reduced visible color gamut on the vivo, they're like chalk and cheese when compared side by side.
BTW, I used to calibrate color grading equipment for the film processing industry and the controls were strict, 18% gray walls, D65 calibration sources, densitometric equipment, Ishihara tests for me, etc. so I'm well aware of the issues.
