Since I teach nature writing and at intervals attempt to practice it, I'm qualified to say that I know where this guy is coming from.
As with so many endeavors, nature writing has become specialized and polarized. On the one hand there is the generally healthy movement from the anthropocentric to the biocentric, from human-focused to world-focused, a movement that Thoreau anticipated late in his life with his more scientific writing. This movement has led to some fine objective writing, but it has also led to many dull pages, exhaustive and occasionally exhausting works. The problem is that most readers are human beings and therefore naturally interested in the human. The driving youthful question that enlivened "Walden" -- "How to live?" -- has been all but forgotten.
Or usurped by the opposite camp. At the other pole are writers with a too easy access to the "spiritual," writers who replace hard-won thought with idealized references to Native Americans and who repeat the word "wonder" over and over. Theirs is a cloying and simplistic philosophy of "nature is good," and they see symbols in every acorn. Nature becomes a kind of bland church, and these writers seem intent on smearing themselves with what Mark Twain called "soul butter." Long gone are the fried rats.
He turned the rant into a book, and I suppose I should read the whole thing!
If I could, I would change the name of my course to "Nature-and-Culture-Writing in the West." But just changing a university course name means going through three levels of committees, and one of them would probably pronounce the name too lengthy for the catalog.
If I can just get students to consider that nature is not "out there" but also "in here," I would be a happy man.