I stumbled on this gem as I scrolled though my notes on the fascicle of Shobogenzo I'm currently studying (Keisei Sanshoku). This is from my notes and not a direct quotation from SO (except the part in quotes at the end). It's entirely possible that I misunderstood him or made a mistake.
Dogen said that the Platform Sutra is not authentic based on the statements of Nanyo Echu (a disciple of Hui Neng, the sutra's author), and contemporary scholars agree. The earliest version and latest version of the sutra are very different - many things were added and changed. Dogen's criticism was about the frequency of the term 'kensho' in the text. In fact, the oldest version does contain this term but not very often. (There is a translation by of this early version by Red Pine)
Q: Should we read these texts?
SO: Jewels and stones are mixed, and it's very hard to tell the difference. Our life is limited. There are so many texts, and we need to choose what to study somehow. We should choose the texts that are helpful in our travels. Even to study Dogen we need to study so many Buddhist texts. Maybe asking the question, "What is the meaning of my practice?" could be a guideline for what to read and study.