Ah well. Visited the dealer today, and one of the sales staff knew what it was. An option in the infotainment system allows opening the boot lid to release the MANUAL sliding boot cover to its first stop.
It isn't even called this in the on-screen settings. You have to find it under Car Symbol> Exterior> Opening and Closing> "Open luggage compartment cover automatically".
On the online manual it might be referred to in Roll-up luggage compartment cover / Settings / but the wording is obtuse.
The sliding cover is hardly a parcel shelf, unless for an empty box. Not like the hatchback car.
So it is not an "Electric sliding parcel shelf", it is an "Electrically released catch allowing partial mechanical sliding of the luggage compartment cover".