The person in question has a relatively rare medical condition, for most people dark mode reduces readability as a darker screen leads to more pupil dilation which causes halation. (which you can test out yourself if you stare at dark mode text and then look at a bright surface, you'll likely literally see a halo of the text).

This is worse in people with astigmatism, which is about 35-50% of the population. Dark mode defaults, as on that website, need to die. Most of the time they're used because people use their digital devices like goblins in a cave and don't light their rooms properly, or configure their brightness settings correctly.