The problem with the Mizmor L’David’s explanation is that you have to be above the arctic circle to get to a point where the day is that short even at the winter solstice, and there were no Jewish communities that far north at a time when the Minhag already existed (we have no idea how old the minhag is, but it is at least 700 years old).
The Mizmor L’David brings us to a far more important point, one which my Rebbe won’t let me be Machmir on (and certainly not Meikel, because he says such a Chumra is so not accepted so it has to be wrong), namely that the 6 hours between meat and milk should be Shaos Zmanios.