My inner cynic...
...reckons that the reason you don't get to see the screen is that this video was shot *after* the external design prototype was crafted from purest Aluminium and *before* the internal gubbins could be crystallised from their vapourware state.
If I were in a bitchy mood, I'd speculate on Apple's marketing team having a patent on that process. Good thing I'm all smiles today, really...