Why Does Iphone Mirroring Not Work If You're Connected To It By Hotspot?

Apple is saying it won't work if the iPhone has a connected hotspot:

iPhone Mirroring: Use your iPhone from your Mac - Apple Support

Same here. A workaround is to enable iPhone Mirroring before connecting to the Hotspot. Yes, it works even without WiFi connected.

I may be wrong, but I don't believe that's going to work. To mirror your device, both have to be on the same Wi-Fi network, and technically when your iPhone is the hotspot, it's acting as the router so it's not on that Wi-Fi network (it's network connection is to the cell network). I'm pretty sure that you'll need to connect both devices to another Wi-Fi network for this to work, though perhaps someone else here will know a workaround.

Regards.