That is exactly why I have to take this article with a grain of salt.
They may have valid points, but they also wrote a book, and this article promotes their book.
I grow weary of anything on the web with an undertone of "Buy my Book! Buy my book!"
What's wrong with that? Read the material critically. Scratch out a few letters on a piece of paper. If it makes sense and you want to go further, buy the book.
As far as advertising goes, this is pretty useful -- it gives me some value even if I don't buy the book, and directs me to a resource that I might be genuinely interested in.
I take potential self-promotion with the same grain of salt, but it's a mistake to dismiss everything this way.
I also did the self-course in their book a couple of years ago, and the results were amazing. This really is a better way to write than what most people have ended up with. The only reason my writing is not better today is that I just don't hand-write enough to keep from devolving to what I learned from school.
The authors have been involved in hand-writing for decades. Anyone with poor hand-writing would do well to consider this or some similar focus on italic writing (there are more than one).
I grow weary of anything on the web with an undertone of "Buy my Book! Buy my book!"