It was great luck for them.
Fact was, the gold must have been hidden in the old castle many years before.
Perhaps it was centuries.
As Jack never discovered the truth of the matter, we cannot explain it here.
Once the gold was divided, one of the Arabs approached Jack, and in broken French asked him who he was.
Jack made up a neat little story, to the effect that he was a traveler who had put out in a boat from his steamer, landing on the island.
He added that his boat had drifted away in the fog.
For this he had a reason.
He saw that they had no idea of the presence of the boat, or of Harry being with it.
The fog completely concealed the little cove where it lay.