Silhouette is important in SC2, making it easy to pick out units. This is what the art direction is talking about - Ease of Recognition, being able to spot a unit out of the background noise. TF2 is used as an example because the silhouettes are so dynamic.
Blizzard never stated that units required to have unique silhouettes. A Brood Lord shares a similar silhouette with a Drone and Mantaling. This is balanced out with the fact their sizes are different.
In TF2, each class needs a unique silhouette, because a Heavy at a distance can't look like a Scout close up. In SC2, your camera never changes distance. Scale can be used as a way to differentiate units in an RTS with a fixed camera.










