The only statement I could find from Apple was from the iOS security guide that states, "it utilizes its own secure boot and personalized software update separate from the application processor." I think we can both agree that's a pretty vague statement, if you have a better source I'd like to see it.

A former Apple engineer said on Twitter:

"@AriX I have no clue where they got the idea that changing SPE firmware will destroy keys. SPE FW is just a signed blob on iOS System Part"

https://twitter.com/johnhedge/status/699882614212075520

Then Apple seems to confirm it:

"The executives — speaking on background — also explicitly stated that what the FBI is asking for — for it to create a piece of software that allows a brute force password crack to be performed — would also work on newer iPhones with its Secure Enclave chip"

http://techcrunch.com/2016/02/19/apple-executives-say-new-ip...

I understand that the boot chain is the only way Apple may modify the behaviour of the Enclave but how would the update be forced? DFU wipes the class key, making any attempt at trying to brute force the phone, useless. If debug pinout access is available, then why does FBI needs Apple to access the phone at all?