Maybe the idea is that flatpak can have better sandbox control over applications running in flatpak using xdg-dbus ie. you can select the permissions that you want to give to a flatpak application and so sometimes it can act near native and not be completely isolated like containers.

Also I am not sure if apptainers are completely isolated.

Though I suppose through tools like https://containertoolbx.org/ such point also becomes moot & then I guess if they move to container, doesn't it sort of become like toolbx?

To be honest, I think a lot of tools can have a huge overlap b/w them and I guess that's okay too