I think understand what you guys are trying to say:
The elbow moves in an arc around the shoulder, regardless of what the forearm is doing. There's only one bone connecting the two, no joints.
Therefore as the elbow moves forward away from the body it will invariable rise with respect to the shoulder.
However...
If I did want my elbow to travel forward along horizontal plane in space, I could achieve this by shrugging my shoulder downward or sinking at the knees.
For myself, I prefer to use the idea of a sunken elbow as a visualization method to get the desired tracking and force than a literal spacial exactitude.