(2.) If the generated servlet really did extend HttpServlet, it would not inherit the HttpJspPage interface, because HttpServlet inherits the implementation of the Servlet interface from GenericServlet, but it does not inherit HttpJspPage.
Who inherits what here? Who gets the house, who gets the car, who gets the kids???
I don't have HFSJ, so I can't explain exactly what the book page is saying, but the requirement is just that the generated class implement HttpJspPage. The container is not required to do so by extending HttpServlet, although it certainly could if it wanted to.