Compare the chief engineer's central line, "Things seem to be worth nothing by what they are in themselves. I begin to believe that the only solid thing about them is the spiritual value which everyone discovers in his own form of activity." Conrad's description of the novel's gestation contains in full the novel's theme that surface facts have no meaning (and wealth no value) except as assigned by the subjective consciousness.
Along those lines, note that "it" in this sentence can refer as easily
to "a valuable commodity" as to "the mere story." In this
reading, what "people say," what may be "true or untrue,"
is that the commodity has value -- and Conrad's verdict is that it has in fact
no value "in itself."