Different goals and different designs. Why are there so many Linux distro?
Snap is proprietary. Appimage does not include distribution and updates. It also doesn’t attempt sandboxing of any kind.
On the other hand, I find appimage very convenient to use.
Maybe I’m being stupid but a trivial way to ensure this is just don’t connect it to the Internet in any way. No SIM card. Cut it off from the Internet after setup, and only connect to a LAN with your chosen services all physically isolated from any internet machines.