Mass Ayoob and others have documented this
Basically total bs. I'd like to see the actual documentation. That's similar to the old Street Survival book that was popular in the late 70's, early 80's that had a S&W lying on the ground with the decock/safety down as an example of a mistake that cost a LE officer his life. The only problem was that it was later admitted to be staged.
While the disconnect might have helped in a few instances, it has caused more problems than it solved and hasn't been seen to be a great item in LE firearms. If it was, you would see it in the Glock line. Even S&W dropped it as a stock item in their M&P line.
Our left handed officers constantly had the seat belt cause the magazine to be slightly released while operating a motor vehicle. It became second nature for those people to just assume the magazine could be released and they would constantly have to check it when exiting the vehicle. A pain in the a*s. While it wasn't a mag. disconnect problem per sea, it did make the firearm non-functional.
As far as removing it when handling prisoners, I made that a violation of our policy. Removing the magazine when you still had two on your duty belt was just plain stupid. Following the policy of multiple officers and securing the prisoner properly does much more for officer safety.
As far as this incident, it didn't say if the firearm had a magazine disconnect. The magazine could have been dislodged through a strike to the firearm by the bat and maybe one round was fired and that was it for that firearm.