The mechanism they are describing here is the emergency one (like if a human is trapped against the machine by something metal and is being crushed - you need to kill the magnet NOW). There is a slower, much safer mechanism for deactivating the magnet that should have been used here but that would require the officer admitting he had made a mistake and asking for help.

Also I just want to point out that the rifle should be considered no longer safe to use unless thoroughly inspected by an expert. In a similar case some years back, the police officer’s sidearm was pulled into the machine. After retrieval it was found that the weapon had been magnetized by the scanner and as a result the firing pin was able to spontaneously release.