My reading of that PDF is that Secure Enclave software can be updated too, it simply does an independent verification of the Apple's digital signature.
So while the Secure Enclave enforces the delay between brute force attempts, Apple could still release an update that removes that delay.
Re:A Message to Our Customers
by fnordfnordfnord on Wednesday February 17, 2016 @08:39AM
(#11117719)
It could also be that the FBI has already obtained the info, or can obtain the info through illegal means and simply want to use the issue to force legislative and policy changes or public opinion changes in their favor, and or just want to legitimize their having the information.
Not to mention that this is for the iPhone 5c. As other comments have mentioned, newer iPhones have the hardware-based Secure Enclave which add to the difficulty of breaking into the phone. https://www.apple.com/business/docs/iOS_Security_Guide.pdf