this seems to mean that they wouldn't have to spend too much time on revision or editing other than correcting a few minor mistakes here and there
You seem to consider revision as error fixing, but fixing the story is another major part of revising. This is unrelated to how someone created the text (dictation, typing, ...) and involves a lot of cut/pasting and reworking passages. Dictation will not cut this down any further.
Your question only considers the act of writing, but not the act of deciding what to write.
So in a way, yes a writer who already knows what they're going to write and how they want the story to go may be able to dictate and revise a novel in under a week's time, but knowing what you're going to write is the lion's share of a good writer's workload.
As a practical example, let's combine your assertion with other figures:
1 You assert that a writer can dicate 30,000 words per day. You do add some time for revising, but it roughly doubles the total time needed to create the book. So let's say that a writer averages 15,000 dictated words a day, including revision.
2 Word count per book of A Song Of Ice And Fire. Let's apply your assertion (padding for revision) of averaging 15,000 words a day.
BOOK | WORD COUNT | TIME TO WRITE
A Game of Thrones | 292,727 | 3 weeks 4.5 days
A Clash of Kings | 318,903 | 4 weeks 1.3 days
A Storm of Swords | 414,604 | 5 weeks 2 days
A Feast for Crows | 295,032 | 3 weeks 4.7 days
A Dance with Dragons | 414,788 | 5 weeks 2 days
Note: I'm considering a five day work week (six hours per day, as per your own calculation) to give you a realistic estimate.
That's 22 weeks and 4.5 days, let's call that half a year (= 26 weeks). I challenge you to write a book series, while dictating, from absolute scratch, mirroring the complexity and lore of ASOIAF in half a year, while also ensuring you make it feel like an original piece.
For reference, books 2 to 5 took about 15 years to write (assuming they were written between the release of book 1 and the release of book 5). And that's not even counting any effort spent towards future storylines when initially creating the lore when book 1 was still being written. This means that, assuming GRRM worked 30h/week, he's spent about 29 times more effort on world/storybuilding than on writing the books.
You can do the same math for other books, but the result will always be the same: any good story will take much more than twice the time you need to write down the letters (regardless of using dictation or a keyboard).