Z is like Fawcett's Holy Grail, in the sense that unlike Indiana Jones, he doesn't choose wisely, and his bad choice basically sucks the life out of him.
Grann himself doesn't make the Indiana Jones and the Last Crusade connection; he has a more Arthurian view of Fawcett and his search for Z. He makes many references to the legend of Camelot, for example when he calls a manuscript "the Holy Grail for the Fawcett freaks" (16.7) and refers to Jack and Raleigh as "'ramrod Englishmen,' each of whom resembled Sir Lancelot" (1.22). Even the members of George Dyott's rescue crew dub themselves the "Knights of the Round Table." (22.20)
What all these references do is add to the legend, romance, and mystique of Fawcett himself, who lived a life almost as dramatic as that of the mythical King Arthur. These references also underscore the idea that Fawcett's entire quest is based partly on fantasy. Sure, the dude knows the Amazon, and he knows how to survive there, more or less, but the power of his fantastical ideas is so strong that he eventually loses himself in them.