Well, given that the Secure Enclave is a processor, I suspect the relevant statement in the whitepaper just means that it's done by software on the SEP rather t

Well, given that the Secure Enclave is a processor, I suspect the relevant statement in the whitepaper just means that it's done by software on the SEP rather than the main processor, not that it's done purely by hardware. I could be wrong. But if I'm right, note that the SEP will take firmware upgrades signed with Apple's key, same as the main processor.