That's a preface by the individual who found this book only after years of research and then decided to post it online. This is not the author of the book talking about their own book.
This is incorrect, this is the preface lifted straight from the 2nd edition pdf on the original site:
"This is the best book ever written on how to practice at the piano! The revelation of this
book is that there are highly efficient practice methods that can accelerate your learning rate, by up
to 1,000 times if you have not yet learned the most efficient practice methods (see IV.5). What is
surprising is that, although these methods were known since the earliest days of piano, they were
seldom taught because only a few teachers knew about them and these knowledgeable teachers never
bothered to disseminate this knowledge. "
> This is the best book ever written on how to practice at the piano!
> This is a work in progress, some sections have not been copied over, there are probably a few formatting errors/inconsistencies, and generally nuttiness. Please ignore or contribute patches on GitHub.
> This is Sphinx adaptation of Chaun C. Chang’s excellent book, Fundamentals of Piano Practice.
I downloaded the book a long time ago as a PDF from somewhere. It also included this sentence and it stuck out to me because it was so weird. The rest of the tone of the book felt in the same style to me so I never second guessed it, but you might be correct.
Probably from the canonical site, http://www.pianopractice.org/, where a more up to date edition is available for free. The second edition (that the submission link is based on) does contain that sentence, so it is from the original author. The third edition does not.
"This is the best book ever written on how to practice at the piano! The revelation of this book is that there are highly efficient practice methods that can accelerate your learning rate, by up to 1,000 times if you have not yet learned the most efficient practice methods (see IV.5). What is surprising is that, although these methods were known since the earliest days of piano, they were seldom taught because only a few teachers knew about them and these knowledgeable teachers never bothered to disseminate this knowledge. "
http://www.pianopractice.org/