The T1 runs an iOS variant because Apple understands that if you can re-use security primitives (Secure Encave from IOS), you have less surface area for attacks

The T1 runs an iOS variant because Apple understands that if you can re-use security primitives (Secure Encave from IOS), you have less surface area for attacks and you have a more secure product. There is absolutely no need for a separate SoC for the lock screen, when the Apple SoC already features high-efficiency cores.