Oh, Hisoka, you tricksy, tricksy bastard.
Hisoka really is a clever dude. They make a big show of how strong he is, and also how god damn creepy he is, but there’s a sharp mind under all that madness. It’s easy to see that he’s been observing every member of the group whenever he can. Even though he doesn’t know all their powers (I think he’s telling the truth when he says he didn’t know Koltopi’s ability until that day), Hisoka has watched his fellow Spiders whenever he could and sussed out their personalities. Hisoka knows he can get the ball rolling by exploiting Nobunaga’s friendship with Uvogin — just produce one kernel of doubt, and Nobunaga will rush in demanding answers.
And, of course, Hisoka is the one controlling those answers.
It’s almost scary how accurate Hisoka is regarding Kurapika’s Nen powers considering Kurapika told him nothing. (And why would he? There is no reason to trust Hisoka any more than he has to.) It’s a clever narrative sleight of hand — we know Kurapika’s powers, but the Spiders do not, so when Hisoka gives an explanation eerily reminiscent of what Kurapika could actually do, it’s easy to believe Hisoka. But, of course, it’s all a ruse to give the Spiders just enough information to give them pause, but not so much that they’re scared off and decide to risk everything by leaving anyway.
How do we account for Hisoka guessing Kurapika’s powers so well? It’s not such a stretch, in my mind; actually, I don’t think Hisoka was guessing at Kurapika’s powers at all. We can assume Hisoka is well practiced with Nen. He knows many tricks and how far Nen can be pushed, and he would of course be aware of Nen’s system of rules and conditions. As we know from the Zoldycks fight against Chrollo, knowledge of conditions that must be fulfilled to use certain powers — and conditions that can be applied with certain powers — isn’t a secret to practiced warriors. So Hisoka simply conjures a vague, reasonable sounding condition that allows the Spiders to fill in the blanks and convince themselves.
That’s the true brilliance of Hisoka’s plan: He barely has to do anything. All he needs to do is pretend there’s something holding him back from discussing Kurapika with the group. Because Kurapika has killed Uvogin, the Spiders have enough respect for his abilities that it’s not too big a stretch to think he could get the jump on Hisoka somehow. And, really, all Hisoka does is say, “Nope” while Chrollo extrapolates and figures out Kurapika’s powers from there. Because Chrollo’s words hold so much sway with the rest of the group, he is basically convincing them as well. Hisoka using the group’s thoughtfulness and tendency toward logical thought against them is damn brilliant.
Given a choice between two contradictory predictions (half the group dies if they fight the chain user, and half the group dies if they leave the city), Hisoka prods the Spiders toward the result that benefits him most. He even inadvertently makes it more difficult for Kurapika to battle the Spiders, because they more or less know one of his abilities. Everything comes up Hisoka here.