The argument isn't that torture never results in a prisoner divulging true information — that clearly does happen on occasion. Rather, the argument is that torture is worse than traditional interrogation and investigation techniques. (As Josiah Neeley noted on Twitter, even a Magic 8-Ball will give you "correct" information through mere chance on occasion.) Torture apologists make grandiose claims about its effectiveness, arguing that it is far more reliable than traditional techniques. Torture is a great and terrible evil, so if it can be shown that it is even simply equal to non-coercive techniques, that obviously implies its use is absolutely inexcusable under any circumstances whatsoever.
In fact that bar is cleared by a considerable margin. Not only is torture much, much worse than traditional interrogation techniques, it also has devastating side effects both for the agencies that practice it and the nation as a whole. The CIA just got off scot-free for spying on its congressional overseers in an attempt to intimidate them into burying the Senate torture report. As Rejali shows, careering lawlessness that strikes at the heart of a nation's constitutional order is common among organizations that torture.
Torture is rat poison for a liberal democracy.