> The credibility of the author is relevant to the author's argument/results/conclusions to the extent that the argument depends on points whose plausibility requires faith in the author's honesty and ability.

Unless you personally have invested the time and effort needed to evaluate his research, you're stuck listening to the people who have. And unless they're all saying the same thing, you have to look at credentials, reputation, and motivation to decide who to believe. While these factors don't affect the quality of his thesis, they do affect it's believability to the rest of us.

In the real world.