I don't see why he would recommend putting the uth on the back wall. The only real purpose for uth's is to provide belly heat. Heat coming from above (like the sun) and below (like a rock warmed by the sun) seem natural, but I don't know about side heat. It might be confusing for the boa.
My uth heats my boa tank just fine. The temp in the room around the cage never goes below 75 and usually hovers around 80. My basking spot is around 85 at the surface (measured ato the few layers of newspaper I use as a substrate.)