This is true - the compatibility boundary for OS X is at the link-against-libSystem layer. The actual interface between libSystem and the kernel is private to Apple and is not guaranteed to stay argument or ABI compatible. (this includes the syscall() function)
Several xnu syscalls are actually paired with a userspace wrapper function that does some extra stuff. Existing syscalls may even become wrapped in a subsequent release if needed.
Several xnu syscalls are actually paired with a userspace wrapper function that does some extra stuff. Existing syscalls may even become wrapped in a subsequent release if needed.