I primarily use dark mode when I am in the dark or in a dimly lit room.
Having any light blasted right into my eyes hurts. And ruins my night vision. And also messes with my sleep.
Incidentally, for most people, this means that - unless they're spending most of their time outdoors - they're not getting enough light.
I didn’t read the study itself, but usually the scientists (if they aren’t pushing an agenda) are aware of the limitations of their studies. It’s the people reporting and commenting on those studies that extrapolate unwarranted conclusions and generalizations from them.