The AppImage doesn't use the portals since it doesn't strictly have any real need to. They're primarily intended to work around issues caused by the sandboxing. It's not impossible that it might be added (people do seem to like being asked for the editor in the way it provides) but it'd still end up suffering from the hardcoded remember functionality.
If you could recompile the xdg-desktop package yourself you could set the arbitrary value of 3 to something ridiculous like 2 billion, which would effectively force it to always ask for your preferences. I do personally think that it's currently unfortunate that it doesn't have a specific "remember this choice" box and I'd like if didn't remember for the users automatically.
For the flatpak, take a look at Launch external editor (Linux, Ubuntu/PopOS, flatpak) - #4 by fnl