> I have no reason not to believe them.

It's not that these people are lying, but it is common that people's subjective experience is biased by their expectation.

If I expect that dark themes are easier on the eye, I'm likely to experience any eye strain that might be caused by a bright theme in an exaggerated fashion.

Similarly, if I were to buy into this "the Pulfrich effect will slow you down" hypothesis, I might subjectively experience a benefit from switching to a light theme.

I don't buy into the hypothesis, so I'm sticking with a dark theme (as well as f.lux cranked up to eleven).