I read this not as a disparagement of Thoreau so much as a lesson about both poetry and prose, how we are incapable of fully describing that which we experience.
In one sense, Thoreau is a liar and a hypocrite, in the physical, the literal sense, in that he was unable to sustain himself (on Walden Pond, I surmise) without stealing from his neighbor, something he fails to report. In another sense he is a failed guide as well, and in this sense less a failure personally but as a representative of man, unable to capture in words all of what he witnesses and experiences.
In this latter sense he cannot be faulted, I think the narrator acknowledges by his tone.
It is, after all, more of the same liquid, the problem with describing what we experience, the problem with truly communicating, one to another, this time poured into the vessel which is the well known Thoreau.
What I find most valuable about it, ironically, is the reminder of Thoreau's life and pursuits, the documentation of which led me for a number of wonderful years along a fruitful journey of my own: if I did not have Kitahdin, I did have Walden Pond, and I had the Saco River, and Mount Madison, and many points along the Appalachian Trail, South River Falls, Twin Otter Peaks, beyond, and deep into the south where Muir took me, and west into the Pecos and the Ozarks and down into Big Bend country.
It did that for me, unintentionally as it might have been.
Take care,
Joe