I would assume that even if the software/cryptography is secure, Apple would have a physical/hardware-based way to access the data. But they can't admit to this as its a big part of their marketing around the product.
I think there may be an Israel-based security company that has managed a hardware bypass. But this was a few iPhone generations ago. Not sure about the latest products.
All the existing attacks have started with at least a partially unlocked phone.
But can you discount a scenario where a hardware hookup and brute forces through all possible numeric security codes? Could take less than a minute. Who is to say there isn't a bypass that allows them to do this? Very hard to tell
As in the investigation into the Pensacola shooting, the San Bernardino gunman, Syed Rizwan Farook, was also dead and no longer had a right to privacy.
In this case, Barr has a better chance of Apple complying, by simply routing the request through China.