A userspace program can completely avoid kernel IPC if it has no intention to expose the fs to other processes. Client and server code can exist within same "app", without IPC, in the same process
There are plenty of existing libraries that do exactly that. This isn't novel to Fuchsia. A good example is GNOME's GVfs https://en.wikipedia.org/wiki/GVfs , which is basically a plugin architecture to the standard GLib I/O routines. (Although as it happens, it still places the mounts in separate daemon processes.)
(And as others have pointed out, while client and server code can exist without IPC, as the names "client" and "server" would imply, that isn't the primary intention. The docs you link say, "To open a file, Fuchsia programs (clients) send RPC requests to filesystem servers ...." And even the terminology of a file system as a "server" isn't novel to Fuchsia; that's the approach the HURD and Plan 9 both take for filesystems, for instance.)
