Here is a snipit from a post from Dr. W. McNelly:
Is The Dune Encyclopedia Canon?
This document originally appeared in the alt.fan.dune newsgroup.
From: wmcn...@ccvax.fullerton.edu (Will McNelly)
Subject: Dune Ency as canon
Date: Mon, 24 Apr 1995 11:52:00 -1000
As I said, it is a work of fiction about a work of fiction,
and it was compiled with FH's full approval and enthusiastic
encouragement. I consulted with him by phone very often, and he and I
had only one quibble - my assertion that Rev Mother Helen Gaius Mohiam
Jessica's true mother - which is contrary to what the books themselves
say where FH identified some shadowy figure as Tanicia Nerus as her
mother. However--REMEMBER that the material in the Riculoan crystals
were stored there by Leto who severely edited and modified and changed
the material to suit his own purposes. Leto was in effect a modifier
of history, so as I told Frank, my assertion about Jessica was just as
valid as Leto's - and he ultimately agreed - thinking that it was the
kind of scholarly BS that academics go in for - and he had a good
laugh out of it.
I always thought the same as you until I read this post.