Story of Norland Plothole?

I just finished watcing the Noland backstory in Skypiea (I'm in episode 190) and there is a thing I dont understand. In the Noland book that we saw in Jaya, It says that the last words of Noland is "Perhaps the city has sunk into the sea!" but in the backstory Noland has no clue where the gold could be and doesnt say anything before dying. I'm pretty sure I missed something or there is a possible explanation but I dont know what it is.