Fang Zheng had been skeptical of Fox's story, especially after Nymph mentioned that the illusion here was created by a magic item.
He didn't doubt that Fox was lying, but rather that there was something inherently wrong with the story itself…
Think about it, the most contradictory part of the story was, a bunch of treasures were unloaded at Golden Bay, but instead of hiding them, they were left out in the open on the dock for everyone to see. Could that be possible? Even that naive Young Lady knew the principle that wealth should be hidden, would a crew that had gained such wealth from their sea adventures act so foolishly?