In my experience, there is a lot of emphasis on waiting until after Tzeith HaKochavim (the stars come out), which is seen as definitely after nightfall, to start the Seder. The reason is that we want to makes sure that the Mitzvoth of the Seder, and indeed even Kiddush, are done definitely at night. See Mishnah Berurah 472:1:1, for example.
But looking at the Shulhan 'Aruch, and even the Raavad (perhaps the former is based on the latter), it seems clear that there is also an urgency to start the Seder (or at least the meal) early enough that people, especially the children, don't fall asleep (see Mehaber OC 472:1 and see this related question). See also RaAVa"D 7:3 in the RaMBa"M, Laws of leaven and Matzah (from a friend, no time to look up right now).
The word used by the Shu"'A is "משתחשך". The M"B there says, "Lav Davka" ("not quite"). Is the M"B saying, "This doesn't mean quite what you may think it means, so don't mistake the meaning here to think you should start the Seder before nightfall; what is meant here is that you shouldn't wait even later and thereby delay the meal," or is he trying to argue with/alter the Shu"'A, and say, "We don't necessarily have to follow this exactly; the most important thing is to wait until nightfall, and then once we get to nightfall don't delay further"? What I mean is, in a place where nightfall is very late, or even in a situation where the children will definitely either fall asleep or be irritable and the experience will not be a positive one for them, could (or should) a person begin after sunset but before nightfall?
If the first interpretation holds, then it seems, according to M"B, there's no option, and he's clarifying what the Mehaber meant, so that there's no mistake at yours or my Seder.
If the second interpretation holds, though, I might think I can start a little earlier if, in my judgment, the quality of the Seder will be affected, since it's important to have the kids awake and participating, and the M"B is just speaking about an ideal situation where everyone's engaged or nobody is young enough to be very much affected by a late start.