Short-answer: heightened esteem of one's own cleverness. It's not terribly dissimilar in principle to the same sort of rigidity you'd find in their most obsessively rigid critics. If it becomes clinically debilitating, it could be a point for diagnosing one of several psychiatric disorders.
If many people did, there'd be a lot more subjectivists out there right now. The fact is that few if any people have the time or inclination not to take things for granted. Your average Joe has no more reason to trust scientists and engineers over soothsayers. If he dwelt on it as darlingly as this truthism guy does in this particular article, he'd probably distrust just about every bit of his everyday knowledge. Same goes for you.
