There is no targeting fat, there is only losing fat. (unless you are talking surgical or lipo type options.
You cant control where you lose it....only that you do. Lower back fat is probably the last to go. It protects kidneys and the body will hold on to that more than any fat.
Honest feedback, is your legs dont look that lean either. My guess is you still had more fat to lose overall. Once you were better leaned out overall, my guess is your back would look better also.
Dieting is hard. I have never done a show or prepped for a show. But I have been an stayed leaner than what you were on stage there with no water prep or 'peaking'. If I ever were to decide to compete, I would have to cut a lot deeper.
"Bodybuilding" means many things to many people. For a guy your age, you are in great shape. But to me, it looks like you are about 6 weeks out for being where many guys are when they step on stage. You look like you are in great summer shape. Not sure on your size (or ht) but from the pic to be really lean, you still got 15 or more lbs to lose.
Ask your coach for the 'hard' truth. Just looks like you need to lean out all over if you want more definition.