In the case of this incident they're using the term very loosely. Generally "off piste" refers to the back areas of the mountain which are ungroomed and seldom skied for obvious reasons.
If you look at the picture of where this guy fell, it's not really "off piste" but rather an area of the slope that's got a lot of rocks on it. It was the semi-fresh powder that probably enticed him to ski down it.....
Brutal!!!
A photo of the actual site and rocks.