Paper vs display at same intensity and colour, sure. But there’s other differences like actual brightness, contrast ratios, colour temperature, etc. which can have subtle effects. I believe one of the biggest issues in terms of eye strain is when you have a display that’s brighter than the surrounding area, and lower contrast/resolution. I feel like the benefit of dark mode is less about the total light emitted, but when the brightness is from the text instead of the blank area around the text it’s more readable because you’re not trying to read through the glare of the background.