If those software updates force it to erase its private key store, though, then it's functionally isolated from that attack vector. An updated enclave that no longer contains the data of interest has no value.

Ok, if that part is updatable you have indeed a backdoor.

In theory it should be possible to make it fixed (which Apple doesn't seem to have done).

Making it fixed just means you can't fix future bugs. The secure approach is to ensure that updates are only possible if the device is either unlocked or wiped completely.