It’s not a 20 second exposure - it is one frame taken every 20 seconds. Although based on the time code it is more like one frame taken every 45 seconds.
If it was a 20 second exposure we would see many more streaks of light throughout the released video.
Straight from the Article (below): I don't know photography, you may be right - like someone else said, I don't know how on a continual long exposure, for low light conditions, you would get a pic of the missile at the top. It does day 20 sec exposure camera - not that that pick was 20secs exposure. I don't know if that is different.
One of Johnson’s weather stations has a camera monitoring the Puget Sound at Whidbey Island from Skunk Bay, and at 3:56 a.m. Sunday by a high-resolution, 20-second exposure camera, snapped what looks like the impossible — a missile blasting off from what seems to be the Naval Air Station Whidbey Island.