It's sounding like that's the problem. They left an opening for this sort of thing by allowing firmware updates to the secure enclave. That basically makes it a fight over whether the FBI can force Apple to use the required key to sign an update meeting the specifications the FBI directs.
Well, i read elsewhere in this thread, that updating the firmware for the secure enclave wipes the private key contained within. Which means you've effectively wiped the phone.
Sure, if they wanted to they could implement a backdoor. But assuming they correctly created and shipped the secure enclave it shouldn't be possible to circumvent it even for Apple.