It might be later in that chapter on the same subject where he points out that once the bar leaves the ground, the worst thing it can do is decelerate, and that the inevitable pulling down and forward of the scapulae with heavy deadlifts will cause the bar to decelerate between the floor and the top position.
I think if you're supplementing deadlifts with other lifts that improve your shoulder girdle stability (the farmers walk specifically) you'll find the shoulder is plenty stable without the need for conscious engagement and retraction of the scapulae. One last thing, as someone who used to retract the shoulder blades for deadlifts (for the record, I had it corrected by a Crossfit instructor): Once you start pulling very heavy weight (I'd say at double your bodyweight or more), the desire to pinch your shoulder blades together, especially knowing they sag or drop to a neutral position in response to the load, makes the lifter more inclined to try to jerk the weight off the ground.
