Heres a way to look at it. Yes it was a long exposure time, but what you are probably thinking about is a long exposure time of an evenly and constantly lit scene. With a scene like that motion becomes blurred as you can see every movement an object makes from A to B. But here the only real illumination is the lightning itself. So imagine a strobe light in a club. You see someones hand in one position, then as the strobe flashes again you see it in a different place. With a scene like this you only see the end result of the movements, not everything in between. So with each flash of lighting a crisp image is produced, and this resulting image is several of those combined into the same moment as we see it.