I think the point is that it is a posture issue, which is why the keyboard analogies were brought up.
I agree with the article though: the shape and texture of the device influences how you hold it. Both my ereader and tablet have textured backs, which make them easy to rest in the palm of my hand or grip by the edge. The ability to hold it in multiple ways also means that I can change positions regularly.
I disagree with the weight being dismissed. Weight has a large influence upon how the device is held. There is a tendency to hold heavy devices towards the center simply because holding them on the edges creates a much larger torque. It also determines the angle at which you hold your wrist, because certain angles are more efficient for your body to maintain. You don't notice this when the object is light, but you do notice this when the object is heavy.