"Most metal stampings go through one or two die tools to produce the final shape. With the Mac Pro though, the challenge is to produce a massive amount of plastic deformation without tearing, rippling or deforming the perfect cylindrical surface."
and, below the figure:
"The results of the first (of between 4 and 5) deep draw stamping operations. Notice how the Mac Pro part is nowhere near the final length."
As I'm sure you know, there's a big difference between a single-use aluminium can, which is both small and thin, and a large part like this, which has to have both structural stability, very high dimensional tolerances, and a good surface finish. That said, you obviously know more about this than I do, so maybe you can comment on the quotes from the article?