Basically, he didn't write it. Even if he input ideas, even if he contributed passages that got incorporated by the other contributors, he didn't write the whole thing, so it's not canon.
The content of things FH wrote, like notes and outlines and even chapter sketches, but didn't publish, or even things that got cut in the publication process (like those chapters in The Road to Dune
)—none of that stuff is canon, either.
Only what FH wrote and published is canon. Anything else is just interesting materials. Anything written by anyone else is fan fiction.
It's actually quite simple.
Actually, I have to clarify something about this particular point, here -- as a practicing IP attorney, I can tell you that, for all intents and purposes, "canon" is defined in a legal sense by the rights-owners of a given property, which in this case is Herbert Properties, LLC. As such, they nominally possess the legal
right to define the parameters of what constitutes the Dune
canon, whether it encompasses the later, approved pastiche works of Brian Herbert and Kevin J. Anderson, or Frank Herbert's works alone.
Just as an example, if tomorrow Herbert Properties decides to declare those unpublished chapters in The Road to Dune
to be "more" canonical than those in the actual completed, published novel, then suddenly those chapters would become canonical. There's really nothing we, as fans, can do about this, apart from complaining about it on our message board forums.
Similarly, the same strictures hold true for other licensed, shared-universe properties: only George Lucas (the still-living creator of Star Wars
) has the power to declare which elements from various works are canonical and which are not in that universe; while Paramount Pictures holds the final say-so in declaring the same for any elements related to the Star Trek
property (despite Gene Roddenberry's attempted control over this area after TNG first went into production).
So...like it or not, the prequels and inter-quels are canonical, but for the record, here, I personally do not consider them to even remotely be such. That's where another big distinction can be made -- "legal" canon versus "personal" canon, which I think many more of us on these boards are apt to use, to distinguish our feelings about the pastiche-works from the original Frank Herbert books.
Now, on the other hand, if you were to say that the KJA/BH novels do not fit into the "canon" of Frank Herbert's own personal output...you'd be absolutely, 100 percent correct. (Which I think is what you were really saying, in your earlier post, there.)