Sokolof's comment elsewhere in this thread: Intro of the book confirms: “It is written in Org-mode, first made by Carsten Dominik and then developed by many other people. Diego Zamboni wrote the Org exporter to convert Org-mode syntax to Markua, expected by Leanpub.”

So the page width probably was set by Leanpub themselves during typesetting. I am assuming that the Markua language is converted to LaTeX, with the page size/layout being set in that language.