Well, first the OP didn't say the audio recorded the sound of the slide racking, they said it recorded the sound of a gun being cocked. Second, if the slide was racked on audio there should be an ejected round back up at the top of the hill, not over on RL's property.
Eta: I'll see if I can find a good source to explain how these SA/DA cocked & locked semi-autos function. If you have used a Beretta M9 or 92FS it's very similar. Basically you insert a loaded mag, rack the slide, engage the safety, holster, and start your mission. If you encounter trouble along the way you draw, disengage the safety, aim and pull the trigger. No cocking, no racking, no fiddling. RA as a military man would know all that and more. I would bet he fired a M9 for the Guard as a primary sidearm thousands of times.