Cuboids can surely be done, he isn't arguing that. He's just observing all the ways in which cylinders are a very "elegant" solution, with a natural fit to metalworking.
I think metalworking has to be the key point of efficiency, because anything that comes packaged in cardboard (cereal, crackers, cake mix, powdered detergent, etc.) is a rectangle.
I would guess that cardboard is less expensive to source and work, so they can afford to take the extra effort to make it a box.
Cardboard's probably easier to form into rectangular shapes than cylinders - if you look at the way cardboard boxes are constructed, the joins are all done by overlapping which doesn't work so well on curved edges.
