This passage and the story of the man as a whole can be taken as a metaphor that Thoreau is showing us, one which we can apply to our own lives. The man from Kouroo elicits from my mind the poignant quote from Ayn Rand, Art is the technology of the soul. No face which we give to a matter will stead us so well as the truth. Here Thoreau echoes many other authors and thinkers in the quite general, almost generic, and universal sentiment that truth is the only steadfast face which we can give to a matter, and that no other attempt to cover something up will stead us so well as truth. Say what you have to say, not what you ought.
Any truth is better than make-believe.
With this passage, Thoreau continues his parade of pro-truth telling language. He also hints at an issue much deeper than just whether what one says is true or not. In the first line he is not dealing just with truth or falsity, but with the dichotomy between real value and superficial pedantry. He is talking about those people that we all know, who when writing a paper, write not because they believe in what they are saying or because they have an actual passion for it, but because they are trying to please another or receive a grade. This type of behavior and psychology is inherent in so many people today that it is sickening, and the greatest concentration of these intellectual second-handers is in our school systems.