Interesting question. The Forms Servlet is basically our interface between the client and the Forms Runtime. Without it, we would have no way to communicate with our backend process (unless we went back to having a stand-alone Forms Server).
As for the doc and its image, please go back and look again. Admittedly, these are old images that date back to when Forms first went web based, but they do paint a fairly good high level picture of how it all works. Specifically, see Section 15.3 About Web-Based Architecture and Figure 15-2. In this image note the reference to the Forms Listener Servlet. The Forms Listener Servlet is actually a subcomponent of the "Forms Servlet".