It's saying that the bias against religion in the sciencey tribe presents a barrier to actually studying religions as sources of psychological knowledge that are better than random speculation (presumably on the grounds that there was some competition between religions, so that religions that were based on bad ideas about psychology died out and the ones that remain are based on better-than-random ideas about psychology)
Sciencey people think religions are wrong, so they are prone to acting like all ideas to come out of religion are bunk. They shouldn't, so the hostility of sciencey people toward religion hurts science.
Sciencey people think religions are wrong, so they are prone to acting like all ideas to come out of religion are bunk. They shouldn't, so the hostility of sciencey people toward religion hurts science.