iPhones can be provisioned with pretty extensive profiles/MDM so I doubt that part. They probably see all the zero-days and decide game respects game. Just not secure enough for such an incredibly insular power structure as theirs, even without the US connections.
I don't get why they wouldn't already insist on Huwawei and HarmonyOS unless this is more of an Inner Party issue where all the high-ups use iPhone intentionally because its more private than Huawei
I wonder what phone Z has been using all this time...
A totalitarian regime can have a different, plausible reason: no sufficient control over said devices.