I have heard that story, and repeated it myself for years, before I went to work in a prison. I have yet to see two inmates training to disarm police officers, or practicing disarming moves, or having mass instruction on any of those things. That's with 6 years on three differant yard, Supermax, Close Custody, and Medium Custody.
That is NOT saying it doesn't go on, but groupings with physical efforts such as that would be noticed, and I have not, nor have I been told of such on any yard I have worked, not unless the basketball games are really clandestine physical confrontation training!
I will pass a request to our secret squirrels if they have seen/heard of such activity, of a nature that I could legally publish redacted data here.
Now, almost all inmates DO mainting a physical fitness regimen that can be quite intense, but many of them, especially in lock down, have nothing else to do, which is why we have seriously intensified out programs, schooling, work programs, etc, to keep them occupied as much as possible, and possibly getting them more inclined to a productive life on the outside. It actually works, sometimes.
As for training, I know several inmates who are ex-military, some with quite intense training/backgrounds, some ex-law enforcement, and some with long records as street brawlers. Some of them are bound to know howto shoot straight.
When I worked an indoor range, I saw many "thugs" come in to "practice", actually practice impressing thier girlfriends, as many of them could NOT shoot past 10 yards. That was where I saw the "technique" of thrusting the pistol forward as it is being fired, perhaps to gain an additional 1-2 FPS? Nevertheless, most would leave proudly with a target full of a shotgun pattern 3 feet wide from ten yards, and tell each other how awesome they are.
So, the vast majority, from my limited experiance/exposure, which nonetheless will be greater than some, is that the typical criminal does NOT practice shoting, unless it's a desert "get together" for partying, and they shoot up the landscape.
The rare one is the thug who CAN shoot straight.
My hopes and prayers for the officer.