If you look into this story a little further, it turns out that there are a few things to consider.
One, this is actually the result of training. The man served two tours overseas, this is quite literally what he's trained to do. Do you have any idea what a gunshot sounds like from far away? Because it's not exactly a clear sound, and there are a shitload of different bullets (and gun barrels, compensators, silencers, sub sonic ammunition as an even further layer) to make different noise. When you're used to being shot at from far away, yeah a sound like this actually does sound like you're being shot at. I could also easily see someone mistaking it for a bullet landing near them.
He also describes experiencing a tingly sensation and thinking something was wrong with his left(? going from memory, lazy) side. It very much sounds like he had a PTSD attack.
Lastly, he resigned during the course of the investigation into the shooting. Not to mention, the investigation into him concluded that no he should not have unloaded his firearm after hearing an acorn hit the car.
Should he be a cop? Fuck no! He likely has undiagnosed PTSD and should be getting help, not putting himself into circumstances where he is much more likely to be shot at.
However this is not the result of incompetence. This man is a military veteran. He will likely be more proficient with firearms than you or I ever will be. You need to stop thinking of cops as pigs in tight bullet proof vests. This guy, and there are absolutely others like him, is not at all untrained.