Secure enclave as per their docs sounds just like their implementation of trust zone.. err "Trust Zone", most likely following ARM specs.

The main difference would be that everyone knows trust zone through Qualcom's implementation and software - as it's been broken many times. At the end of the day "its just software" though, which runs on a CPU-managed hypervisor with strong separation ("hardware" but really, the line is quite a blur at this level).

What that means is that you need to be unable to update the secure enclave without user's code (so the enclave itself needs to check that) which is probably EXACTLY what apple is going to do.

Of course, Apple can still update the OS to trick the user into inserting the code elsewhere, then FBI to use that to update the enclave and decrypt - though that means the user needs to be alive obviously.

Past that, you'd need to extract the data from memory (actually opening the phone) and attempt to brute force the encryption. FBI does not know how to do this part, the NSA certainly does, arguably, Apple might since they're designing the chipset itself.