The FBI would be best off hacking it themselves , then if they can, not letting on .
For something to be totally unhackable seems impossible, but the difficulty can be extreme and so be practically impossible.
To me this depends a lot on how it is physically built.
If most of the memory is on a different chip than the CPU then the CPU can be separated from the memory and decryption would be merely code breaking because the command to erase would never be triggered by the new CPU.
It might even be simpler than that, suppose the command to erase was known, and the pins this had to energize were known, this connection could be physically cut and a circuit interposed that would pass any signal other than the erase command.
I think the FBI should buy a gross of these phones and play around with this sort of idea.