chahahc I think it might make sense that brighter ambient light can reduce symptoms. The data on the effects of flickering light and epileptic seizures suggest that the percentage of the retina exposed to the light might be even more significant than the intensity of the light since closing one's eyes can actually make symptoms worse because the pupils are dilated more when the eyes are closed so a greater percentage of the retina is exposed to the flicker (see IEEE recommendations on LED light referenced below). Whether the adverse affects of the monitor are caused by flicker, dither, color, or something else, it could be as simple as the fact that the brighter the ambient light, the more the pupils contract, so the smaller the area of the retina that is exposed to the effect. Additionally, if flicker is part of the issue, then if the ambient light doesn't flicker (sunlight, incandescent, and even fluorescent light doesn't go completely off when it flickers), it could help to provide illumination during the dark part of the monitor flicker cycle, so that the eyes perceive less extreme bright/dark differences.
Since migraine is thought to have similarities to epilepsy and, at least in my experience, my monitor issues and LED light-induced migraines seem to be linked, all of this might be part of the same continuum of oversensitivity of the brain to certain stimuli. If the monitor sensitivities are anything like migraine triggers (which vary widely between patients), it is possible that different visual stimuli are responsible for the sensitivities in different people. Although it would be really nice if there was a one common cause that could be identified and then fixed, we might be dealing with more than one cause. Or, causes could be connected - flicker and dither sensitivity might be connected, just as sensitivity to flicker and sensitivity to viewing certain patterns seems to be linked in epileptic patients. Since flicker can create patterns on the retina, that could be a link. I think what is common to all of our experiences on this forum, though, is that our brains just don't like certain unnatural stimuli that we would have never encountered in the natural world. It's kind of strange to think that a heightened perception of modern technology gives people disabilities.
Note that all of the data referenced in this IEEE report - see p. 23 of summary - pertain to testing of epileptic patients long before LEDs existed and/or testing the effects of flickering fluorescent lights. None of the data are actually for the effects of LED lights (since no published data on the biological effects of LEDs on humans exist), but the recommendations for LED lighting are just "best guesses" that were extrapolated from the fluorescent data. I suspect based on my own experience that the recommendations aren't stringent enough since industrial LED light arrays with several hundred Hz flicker are much worse for me than single LED bulbs that flicker at 120Hz - although I don't know for sure that flicker is my problem.
IEEE report 1789: https://standards.ieee.org/standard/1789-2015.html
Summary available here for free:
There's also been some work from Arnold Wilkins at the University of Essex (a lighting expert) on the brain's response to different kinds of patterns - we respond differently to irregular "natural" patterns (think of tree branches in a forest) than to unnatural regular repeating patterns and migraine patients can have more extreme responses to the unnatural patterns than other people.