Hmm, you're right. It looks like some older devices were able to be jailbroken into a state where the Secure Enclave was in an untrusted state, so you couldn't use authentication, but the encrypted flash was still accessible as long as you didn't have a passcode set. I thought even that wouldn't work in an untrusted state, but it appears I was incorrect.

(I was reading recently about the SEP's behavior after a memory authentication failure -- it completely stops accepting requests until the processor is reset -- so I thought that applied to trusted boot violations as well. Seems not.)

I suppose that also explains why every time I open Settings, the Apple Pay section is missing for a moment before it shows up; the OS is apparently able to function without it. Interesting bit of trivia.