Well, Wooten himself points that he will "anchor" the movement at the wrist OR elbow OR thumb joint OR even shoulder. Chuck Rainey also professes slapping mainly from the elbow.
The author points, after all, that the double thumb approach differs from the typical slapping style, hence the provided video examples.
That said, the book is done rather good. The technical development is set up progressively and gradually adds new elements. What I likes especially was the application of the technique to play the bass lines to some real world songs, appropriately chosen for each phase of the technical development.
Also cool are the study examples that attempt to show the stylistic approaches of various players to the utilization of this technique (Wooten, Caron, Miller), plus, the author manages to squeeze in some applied music theory in the form of scales and triad arpeggios.
While at the first glance this eBook may not look like much with 30 pages, it will take you a while to digest the presented material to some proficiency level. On top of that, having the videos to demonstrate some technical details is always a plus.
All in all - pretty good work