your statement is why it isn't that short. The fStop number is high enough to grab both sets of ID numbers, with 315 being the focus point and knowing the 2/3 rule regarding depth of field. 
Long exposure or not, you must remember that each of those lines represents many many tracer rounds fired on path, not just one. (in order to grab that exposure) As such, even if that took place over 17 or 19 seconds, thousands and thousands of rounds may just have been fired from hundreds of guns.
Actually Llogan I'm thinking each of those lines represents a single tracer round; you can see the cone of fire in the slightly different angles, and it's unlikely the guns were static during firing. Also there are at least three near vertical short artifacts above a/c number 315, not sure what they are but the fact that they are all roughly parallel suggests hot debris falling under gravity. Whatever they are, they would have to be moving extremely slowly (or a very long way away) to leave such a short trail at anything like 17 or 19 seconds. And is that smoke in the lower left corner? If so it's been captured almost as if by flash (it may not be smoke, hurried processing can cause similar effects).
As for the depth of field, not an issue here. Even though typically a photographer will use the widest aperture available for 'long exposure' night work, and I believe this is the case here, depth of field at this distance to the subjects wouldn't be a factor with the standard camera/lens combinations likely to have been in use. I wouldn't be suprised if this was a half second exposure.... doubtful the photographer set up a tripod in the midst of that mayhem. More likely had the camera with him on the way to the shelter and thought he may as well try a hand-held of the firework display. I've won bets shooting blur free handheld at half a second, you pick up a few useful tricks as an RAAF photographer. Personally I'd have already been cowering under my camera case in the shelter
