Didn't they have the design only and none built at the time of reveal? Like the Apple Watch? That's probably the safest way no products, no leaks.
Didn't they show a video of them being built at the Texas factory?
The iPhone is a mass produced device and given the volume and fact that much of the supply chain is overseas, it's difficult to contain leaks.
When all of your Mac Pros are made at one location in Texas, it's far easier. Even if they've already been built.