Presumably it was just the outer layer. As one of the goldbugs around here once said, gold foil can be about 1/1000th of an inch thin.
That is true: Gold is the most malleable of metals.
1 g of gold can be drawn into a wire of ~1.75 miles length!
Didn’t see any mention of it being an outer layer only, though, and how then would they determine the weight with any certainty?
Anyway.... nice find. 😀