Imminent does not mean it has to happen.
Individuals are authorized to use deadly physical force when they reasonably believe it is necessary to, defend themselves or a third person from the use or imminent use of deadly physical force...
Action is not required. Only a reasonable belief that an action is imminent.
It's up to a jury to decide whether the belief in this case was reasonable. There certainly isn't the cut and dry "let's hang him now" evidence that you claim.
Well then one can reasonably determine from the parking lot photos and the photos of the cars, that Officer White fired into her car when it was behind him and moving away from him. Else we would see bullet holes in her rear glass or trunk and not 4 holes in her windshield. From the position that the cars ended up in, which you can see from the bright orange markings in the parking lot photos, she was in reverse, moving away from him, thus a retreating "threat" at best. Simple trigonometry will be able to show this as we know where the cars were, and the bullet holes in the windshield and where they landed can be plotted.
http://www.nbcsandiego.com/slideshow/news/15904630/detail.html
Where did I claim that?