I think Christensen's statements on both iPod and iPhone stems from theories and sweeping generalizations on how technology field works. But with Apple, the devil is in the details. His argument is not taking into considerations things such as

1. Control of the supply chains so that other players can't get their hands on similar hardware.

2. Singularity of Taste coming from Jobs and Ive that blended with the hardware.

3. Software, that is very hard to replicate without years of effort.

The lesson is, theories are theories and Christensen is not god.