It's not. If any ebook had all the information you needed to make money online, the author would just use the techniques himself and scale them instead of putting them in a $7 ebook.
Perhaps the technique he describes in his ebook is to sell $7 ebooks, which he is apparently doing.
You're right. I believe the premise of the $7 ebook is that $7 is the "breaking point" price that people can buy something they're unsure about and not be upset that they've wasted too much money.