Could it be the case that Apple is leveraging TrustZone but with a customized L4 kernel? Or is it confirmed that the Secure Enclave is a custom IC designed by Apple? I wouldn't be surprised if it's the former as it becomes much cheaper to implement the required security features.
Edit: Check out this previous discussion on this exact topic: https://news.ycombinator.com/item?id=8410700
By default no SW runs on HW. "Mobicore" (now called "Kinibi" from Trustonic) is based on L4.
No, 2012 was when Trustonic was formed from competing TEE vendors: ARM, Gemalto, and Giesecke & Devrient.
TrustZone has been around since before that. TI OMAP were front-runners of using it.