In boarding closed shoulders mean exactly what wrathfuldeity
Another way to think about is your board is a doorway and your shoulder and hips are the door. The door is open when from left to right side of your body is not inline with the nose and tail of the snowboard. For example: I ride with regular stance(left foot forward). So my door is closed when my left should is pointed in the same direction as the nose of my snowboard and my right should is pointed in the same direction as the tail of my snowboard. This is the easy way to determine if I'm in the proper stance. Because my angle of binding on my left foot is a positive 18 degree my shoulders will vary 18 degrees from the nose of the snowboard when I am riding closed(i.e. snowboarding reference alignement Shoulder, hips, and knees are perpindicular to the front foot). When my shoulders move from this alignment my door is opening or closing in the direction of a certain edge toe or heel side. Typically when we say open the lead shoulder may be aimed at a higher angle then my lead foot. In this case my shoulder's would be open if I pass the 18 degree alignment meaning the front of my body is aimed more to the nose then to the toeside of the board.
The picture you posted deals with posture or bend in the spine. Being hunched over could have an effect on which edge is being pressured(i.e. another reference alighment : Weight between the feet and over the working edge), but most likely in this case is not a main cause of the board kicking out. The board kicking out probably means there is a pivot effect on the boards interaction with the snow. A twist in the spine could cause a rotationary force with lack of pressure which can cause unattended pivot in the board.
Hope this clears up the statement of "open shoulders".