I think I got it. Of course, all supernodes need to communicate with each other and share search requests, to allow sharing between clients which are attached to different supernodes.

It's like a step towards the eDonkey2000 concept: There, servers only manage users and search requests, but don't host files. Here, a supernode is like a two-in-one request server and user client.

This concept I would expect to give a good boost to the Gnutella network - when many clients know about this technology.