Does Starlink have a mode where you can manually tell it its lat/long? For a semi-stationary receiver, surely that would be a better way to bootstrap a connection.
One could use a GNSS emulator and this modification to test how the system performs under various GPS fault conditions to quantify it. My hunch is that it's more of a regulatory check than an operational requirement. I suspect the signals from the Starlink satellites themselves are sufficient for the terminal to derive timing and position without having to rely on GPS at all.
Highly likely the GPS module inside the Starlink device has this ability. Whether or not that functionality is exposed is up to the Starlink engineers I suppose.