> Is your concern the secure enclave in an iPhone is deflatable, and in such a way as to allow key extraction of device unique seeds it protects?

This is a safe assumption to make as the secret bits are sitting in a static location known to anyone with the design documents. Actually getting to them may of course be very challenging.

> Do you have any literature or references where this is known to have occurred?

I'm not aware of any, which isn't surprising given the enormous resources Apple spent on this technology. Random researchers aren't very likely to succeed.