Agree with this point.
IMO, the "average" gun owner has a pistol or two that sits in the back of a closet collecting dust. Once a year (or less) they get the bug to shoot so they dust off the gun, buy some ammo and head to the range. When they get home the gun returns to the back of the closet (without being cleaned) until next year. I don't feel safe shooting next to these guys at the range as shooting is always a novelty to them.
For what it's worth, even guys who consider themselves "regular" shooters only go to the range a 2 or 3 times more than the "average" guy. They may have more guns (an AR, shotgun, etc) so they spend a little longer at the range and shoot different guns, but still shoot infrequently.
From there you get the more serious "gun guys" who go to the range on a weekly (or a few times a month) basis and probably have experience in club or formal matches.
So where does that leave us? Like everything else in life, the more you practice and do something, the more proficient you'll become. That means cops who are "gun guys" and regular citizens who are "gun guys" will be the best shooters. The "average" and "regular" shooters have cop and regular citizens in their population as well, and they will shoot accordingly.
I'll qualify my VERY over-simplified rating system by saying it relates only to shooting a gun (accuracy, timing, reloading, moving, etc, etc).
I won't get into the combat mindset portion of the debate as I think there are likely buckets for that as well that both civilian and cops would fall into. Going through, or not going through a police academy may not be the best measuring stick for a warrior mindset, but that discussion will stray in a LOT of different directions...