You do have to be camped there in order to stay inside after they lock the gates at dusk. If you observe, count on the ranger coming by at some point to ask what you're doing. As long as you're camped and don't appear threatening, you should be okay. The campground and the lake area have some lights. To escape them, I go to a parking lot which serves a picnic area.
Trees there block off all lights.
Horizons are okay there. An alternate Broome County park for observing is Upper Lisle Campground. This is also very dark, though it's also open, making you vulnerable to Coleman lanterns and campfires. Also, it can be incredibly dewy, as the campground is on a peninsula jutting out into a lake. There is a small quarry nearby which is dryer and darker, but is sometimes used as a party site.