Not sure what contradiction you mean. Human bullshit detectors are being gamed all the time (and had been since forever) but they are still more effective than anything contemporary computers could do. Unlike computers, people can go and see events themselves, ask questions to witnesses, ask questions to experts, self-organize, google, bing if google returns crap, lookup official deadtree records and generally do quite a lot of things cheaters would have hard time anticipating in advance.
The very modern fake news phenomenon is a notable example of human BS detectors being successfully gamed by, for example, small shops in Macedonia that generate large volumes of nonsense that confirms their tribal biases and harvests ad revenue from viral sharing. The humans they trick are operating with media literacy skills that were adequate a decade ago but are not today. There is certainly an arms race here.
