An enclave is defined as an area within a territorial boundary. So an exclave is an area outside of a boundary with ties to the main territory
Essentially, Apple is trying to realize the security advantages of a microkernel without tossing the monolithic aspects of XNU.
“In iOS 18, exclaves refer to specific resources that are separated from the main iOS kernel (XNU) and cannot be accessed by it, even if the kernel is compromised,"
Pretty neat hardening of the OS. Decent write up that isn’t too deep in the weeds.