For my application, I don’t need the command-chain entry. I assume it has something to do with the desktop entry in plugs. Maybe I am doing it wrong, but it works for me.
The desktop-launch interface that this document is describing is not the same as the desktop-launch script used by many graphical/desktop snap packages. The interface above is about granting permission to a snap package to launch executables from the system or other snaps from within its strict confinement. The script of the same name used by desktop snaps, however, is unrelated and is used to configure the runtime environment of the desktop snap so that the app can show it’s user interface and access things like audio, 3d acceleration, and gnome or kde services.