You are misparsing the sentence.
That's why I don’t doubt it.
What is it in this sentence? It, as a pronoun, has an antecedent, the thing it refers to. Typically, as is the case here, a pronoun’s antecedent is the most recent thing mentioned that the pronoun could refer to, here that Lyman Zerga is
One of the biggest.
Benedict and Eddie aren’t discussing whether or not Lyman Zerga is actually a businessman, rather than a scam artist. That possibility doesn’t seem to have entered into their minds.
Rather, they are discussing whether or not Lyman Zerga is actually a big enough deal to justify a private meeting with Benedict. Benedict is a powerful, wealthy, important man—particularly in his own opinion. His time is valuable, and more than that, he is not the kind of man who gives away personal favors, such as a private meeting, to anyone who asks. In fact, he is the kind of man who gives personal favors only to the absolute highest echelons of society.
So Benedict’s concern here is whether or not he should spend time on a private meeting—if Zerga isn’t a major player, doing so is a waste of time and makes him look foolish. But if Zerga is one of the few people important, powerful, and wealthy enough to be worth meeting with, refusing a favor, particularly a relatively minor one, could be a considerable offense. If Zerga is the kind of person Benedict wants to meet with, he’s the kind of person who talks to people Benedict wants to meet with. Having such a person saying good things, feeling gratitude towards Benedict, is important and valuable to him. Snubbing him could tarnish his reputation. But being important and powerful himself, he has a lot of people asking for his time; he has to make sure he gives it to the right people.
This is the kind of calculation that Benedict has to make constantly, all day every day. Providing him with the information necessary to make that kind of call correctly every time is literally why Eddie has the job he does. Based on his contacts, Eddie understands that Lyman Zerga is a businessman, mostly dealing in arms. This much isn’t being questioned. The question is whether he is “one of the biggest.” And in the arms business, the biggest arms dealer you’ve heard of is almost-certainly not one of the biggest out there—the really big movers in that field keep themselves anonymous. Or, at least, so Eddie believes.