The article mentions that for their purposes 1/6 of 6 planes is the same as 1 full plane. However that is not the case. The remaining 5/6 of each of those planes is restricted to cargo with a high volume to weight ratio. So in reality, needing 6 planes does effectively restrict the cargo of six planes not just one. This in turn causes a much greater disruption than what the articles calculations suggest. For example those needing to ship similar freight to the phone (other electronic companies) would not be able to use the remaining space.
Very good points. I think there are enough other estimates that the volumetric weight calculation isn't a huge impact. We could easily be off by 25% on the number of phones Apple ships to the United States, for example, since they don't disclose this number. But yeah, you're right.