I think they were pointing out how some people's perspective might indicate a different "darkest day". As in how for some it might seem that an earlier sunset is the darkest day since they might be sleeping in past sunrise on that day. Or how for some who get up early might note that it is longer until the sun comes up. But it may just simply be that, yes, the day with the least amount of daylight hours would indeed be the "darkest day".
However, I sense what you are looking for is something very similar to the question that was on my own mind and which is the reason I am here as well; that being that there may be some other atmospheric or geometric phenomena that would decrease the amount of visible light to something less than the actual numeric time that the sun is visible on the shortest day of the year. To my knowledge, there isn't anything that would cause the amount of photons to be present to be less than there should be except things like mountains or weather.
I think they were pointing out how some people's perspective might indicate a different "darkest day". As in how for some it might seem that an earlier sunset is the darkest day since they might be sleeping in past sunrise on that day. Or how for some who get up early might note that it is longer until the sun comes up. But it may just simply be that, yes, the day with the least amount of daylight hours would indeed be the "darkest day".
However, I sense what you are looking for is something very similar to the question that was on my own mind and which is the reason I am here as well; that being that there may be some other atmospheric or geometric phenomena that would decrease the amount of visible light to something less than the actual numeric time that the sun is visible on the shortest day of the year. To my knowledge, there isn't anything that would cause the amount of photons to be present to be less than there should be except things like mountains or weather.