This is not true. When you manually disconnect an interface it will not re-connect again on its own. If it does this is a bug that we will fix.
Still, I agree that using interfaces for this is not best because it interferes with applications and may prevent them from working (even if they otherwise work while the system is offline).
I know it requires some more hoops to jump through but I would encourage researching network namespaces and using a 3rd party tool to launch a snap app inside a network namespace that is disconnected from the regular internet. This has the advantage, as compared to a firewall, that is is easy to scope to a specific snap.