No, it does not rely on "undocumented proprietary protocols". The document you link clearly states that external flash contents are encrypted with a key that resides in the secure enclave, entangled with the user passcode. Furthermore the secure enclave has guards against brute forcing the passcode with an escalating time delay.

What's different in more recent phones (>=A7 processor) is the secure enclave enforces that time delay, as opposed to the operating system, which is the reason why this brute force attack works on the 5c/A6.