Religious faith, arguably a form of mental torture, is just as undeniable as physical torture. He uses "undeniable" in the same context he uses "imperative".

So quite obviously, "some people" is referring to those "believers", including Kurt Wise himself. From Dawkins's perspective, Wise's doublethink is a result of a mental torture. That torture is so undeniable that even a Harvard-educated geologist had to give in.