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I read this somewhere, years ago, and at first could not understand it.
But as I continued writing, I found it was true. Writing is a kind of thinking (or can be).
One consequence is that a writer's writings are sometimes much more intelligent than he is in 'real life' - Kurt Vonnegut admitted this candidly - that he came across as smart in his writing, but not when people met him. This happened because Vonnegut could refine the written word, work on it to increase the concentration and wit - but could not do this in real-time.
Another example is JK Rowling who - with Harry Potter - managed to write a very extended and complex plot across seven books while creating characters people care-about and firing-off lots of fantasy ideas. Yet in interviews she comes across as incapable of having done this.
On the other hand, some writers are pretty much the same person on the page and in real life. I used to know the Scottish author Alasdair Gray quite well, and his conversation was very similar to his prose style. And Gray's prose style was pretty much the same in letters and diaries as it was in his novels. It may be relevant that he was a 'natural' writer who won a national competition and published a short story in his childhood (The Star) - and this was good enough to be re-published with adult work in Unlikely Stories, Mostly.
The contrast can also go in the other direction, with writers who are good enough at writing to become known as such - but who by all accounts were much better conversationalists: Samuel Johnson and Oscar Wilde spring to mind (none of Johnson's works are in the mainstream canon of English Literature, and only the Importance of Being Earnest from Wilde's ouvre).
I would guess that the explanation is that for some writers like Vonnegut writing is thinking - which allows them to extend their natural oral capabilities; for other writers (Alasdair Gray?) writing is transcription; while for others (like Wilde, perhaps) writing was a distraction from living ("I put all my genius into my life; I put only my talent into my works."); or even an actively unpleasant activity - as Sam. Johnson said: "No man but a blockhead ever wrote, except for money."
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