If it were just about the value of the information conveyed, people would do their own research when a person is "brutally honest" with them, and decide whether or not that information itself is credible.
The confusion lies in the fact that in a community like HN, that worships logic (and also success), those things that make you "credible" are what make you a high-status individual. That means even the statements those high status people make which are NOT credible themselves will be accepted, or only gently rebuffed. (Which is a big reason why you don't see much criticism of pg here, or when you do, it's very respectful, vs other authors, even when he writes something ridiculous.)