An appropriate analogy which came to mind when reading the piece is the Encylopedia Brittanica versus Wikipedia. Many questioned the credibility and accuracy of Wikipedia (an open source encyclopedia) at first. I think the last stats I saw (some time ago) was that there are now fewer than three errors in Wikipedia for every error in Encyclopedia Brittanica -- and errors in the former tend to get corrected more quickly than errors in the latter.
I think the concern -- both with Wikipedia and open textbooks -- is not so much about small errors. It is more about authoritative voices and the vetting of content by authoritative voices. There is a fear (real or perceived) that content that is not vetted by field experts runs the risk of being inaccurate in fundamental ways -- e.g., excluding evolution as a valid scientific theory over or along side creationism in a biology textbook. Conventional wisdom (which is often wrong) or political agendas can more easily be embedded into texts if there is not a vetting process by trained or experienced experts within a field. Of course, some in the scientific or academic community might say this happens with traditional texts as well, because new, novel, or as yet unproven theories often have great difficulty getting published. Certainly there are examples in the open source space which address this concern around vetting content -- Flat World Knowledge and Connexions being two very positive examples.
Maybe we have to come to some agreement what we mean by "quality." That said, the opinion piece raises some interesting ideas and perspectives. Should open textbooks be held to a different standard? Probably not. One could ask if for-profit universities should be held to a different standard too -- with an equal amount of debate to follow. For a new technology to prove itself and be adopted in the marketplace is not an entitlement -- it must be earned. For open textbooks that means being held to a higher standard until perceptions and reality are aligned at a point favorable to the technology.
The open textbook community should welcome and embrace that higher bar as a challenge to create even better products -- exceeding the products produced by traditional sources. That in turn forces those traditional products to either improve or exit the marketplace. Remember that it is not always the best technology that wins--but often the technology that is able to capture the greatest amount of market share. For open source to be successful in the long term, the movement must focus on continuing to build market share. Part of that will happen by demonstrating open source meets a higher standard than traditional textbooks, if such is the case.