That also makes no sense. When you compress the spring the truck will just sag closer to the axle. So with your process you now have the axle lifted off the ground and the frame closer to the axle yet no space between the top of the spring and the cup where you need to slip in the spacer.

 

Rice, I'd guess he lifted the truck by the axle, put stands under the frame, compressors on the springs, and then dropped the axle. Just a guess, but it makes perfect sense to me.