Demandred could have studied the art of fencing before the war. After all Sammael was a World Champion at the sport. As far as I remember Demandred was a general for the light and turned at some point in the actual war so that would suggest that he was not out serving the Shadow during the collapse. We know that violence did rise during the collapse that that took 100 years plus. Perhaps Demandred was invovled in policing actions during the collapse or once the world became morev violent he may have started working out at his fencing a lot more carefully as it was now something that was useful before the actual war started.