The tester is saying that the earth is open circuit, current can't pass from live to the earth wire.
The tester likely has roughly this schematic (found with a quick search):
View attachment 16074
So I can't see how the RCD will work to protect appliances! It's possible that still connect neutral to chassis when in V2L mode, and they bring the earth from the car's chassis to the box, but don't connect it to the sockets' earth pins. They could use that earth wire for the test function, and indeed in this configuration the RCD would protect protect occupants that touch live and the car chassis at once.
But why leave off the connection to the sockets' earth pins? It beggars belief. Maybe they leave the earth lead out altogether to save some pennies, and test against neutral. But that's hardly a real test. In effect they would be saying "there is no way that you can cause a real current imbalance with this, but if somehow you found a way, this is how the RCD would react" (by tripping, if it's working).
[ Edit: Added verbage to clarify some points. ]