He is the creator of the Vivec himself, of the Sermons, the Monomyth and other Creation Myths, nearly all metaphysical stuff, mythical stuff and more. Who, if not him, would know these areas better?
So, for example, if Tolkien was asked about the future of his heroes or the Middle-Earth, and he'd answered them, but not published officially, I ask you - would everyone treat this as canon or not?
Second, "canon" =/= "truth". So if you call one source canon, it doesn't mean that this source is totally true or at least half true.
Third. Some of his texts were to be published in the Pocket Guides, but has not been published for just economical reasons. How could one throw this stuff away just for that reason?
Fourth. Canonical in-universe sources are widely biased and written in-character by people educated in the lore (far more educated than fans). Non-canonical in-universe sources are widely biased and written in-character by people educated in the lore (far more educated than fans). Where is the difference?