The case is a test mule.
Like any good terrorist organization, Apple is fully compartmentalized.
No one working on the internals has the final case design and vice versa.
If the back is really a new material, then it is on there to test, but that frame, is just not a final product. As in 0% chance.
The funny thing is that some (former) Apple engineer was out testing the thing in public, leaves the bar and doesn't think...
"Hey where's that prototype iPhone I'm supposed to be *real* careful with."
Good resume material: I'm the dumb*ss who lost the iPhone prototype, you hiring?