I don't think iPhone Mirroring requires both devices being on the same (or in fact any) Wi-Fi network. It does however require them to be signed in to the same iCloud account.
Interesting that it works no matter the WiFi. But it’s still not tethering.
Lots of people who are entitled to a corporate smartphone also have a single phone with two sims for work/personal, because of the same reasons: cheaper, more convenient, large data plans on corporate device. These devices are MDM enrolled and the company will at least check what apps are installed.
