Reminds me of The End of Average, [1] a book about human variation. It talks about how the Air Force tried to design a cockpit by averaging various measurements, only to find that their pilots weren't actually average.

Your point is especially apt here, where there is a possibility that the technology could be helpful for a subpopulation, and act somewhat like an assistive technology. It is very tricky to assess the utility of assistive technologies (I work in this field and have asked many experts how they do it) because the average impact isn't the most important thing. It doesn't matter if high-contrast mode is bad for 90% of people, if for the 10% who would actually use it, it's helpful.

One way that the study could have been optimized is if participants had been asked (after taking the test, to avoid priming) if they have any specific reading challenges. That would have helped identify whether there appear to be any subgroups that disproportionately benefit from the approach.

1: https://www.amazon.com/End-Average-Succeed-Values-Sameness/d...

This is also described in the book "The End of Average" https://www.amazon.com/End-Average-Succeed-Values-Sameness-e...