I see where he's coming from, but I don't think he's correct. SAS training (as I understand it) puts you physically in a position where you shoot with an actual gun at humanoid shapes who fall over. Videogames put you in a position removed from the action, in a chair or bed, pulling a button on a controller to kill digital representations of humans, animals, mythical beasts. It's an important distinction to make, the difference between a controller and real gun, and the difference between watching it on a screen and being physically there.
Also, there's the element on outside knowledge. If you joined the SAS, you know youre training to kill real humans; if you're playing CoD, you know you are using a simulation to blow off steam. It takes someone very screwed up in the head to mistake the simulation for real life.