Jack took a huge risk, not that he showed it on his face. Through Lex's vast exposure and experiences, he had learned random tidbits about the universe and its many organizations. In some areas, he might know secrets not known to most, while in others he might lack common sense that a member of a traditional universal organization might have.
He had learned tidbits about the Humanoid Alliance, as well as the Celestial Court - not to mention the little secrets a certain monkey guest shared with Lex. He heard of conspiracies, and got a sense for how deep and long lasting they were, as well as how important it was for all organizations to keep face.
To list all the random stuff Lex knew would take forever. What mattered was that Jack used those bits of information to make a wild guess, and take a risk. He had no idea if, as the strongest fairy, he qualified to become the leader of the fairy race. He didn't even know if most races had leaders.
