You are under the misguided assumption that writing is just the act of putting words on paper. The verb itself certainly has that meaning, but when applied to the writing of books, there is also conceptualizing, planning, outlining. Many works of fiction have at least the same number of words in notes and ideas.
And then there is the word that I'll only mention in a hushed voice, for it is the bane of every writer, especially those not long engaged in the art:
Discarded chapters. Rewritten chapters. Re-rewritten chapters. Dialogs redone, characters removed, then re-introduced, then removed again, then rewritten to be a different character. Plot lines reworked. Language and grammar corrected, then corrected again, then reverted to a previous edit, then fixed by the lector, re-fixed by the author, discussed with the publisher...
For every word on paper, there are multiple words in the trash can, the edit pile, the notes stack and the "to be discussed" list of lector or publisher.
You might be able to put down 5,000 words in a day, if everything is already clear in your head. If you actually try to do that, you will find that even when you are just trying to report a recent event that you witnessed yourself, you will find that difficult. Right after dictating one sentence, you will think of a better way to say it.
That the assumption is nonsense could also have been clear from another perspective. If you look at audio books, 10, 12, 15 hours are not unusual running lengths for books. That is the time it takes to read the thing when it is already written. And it does not include the time for mistakes and re-tries, which are edited out. It also doesn't include correcting mistakes which are still common in dictation.
Given that it takes this time to read a book when finished, it is highly implausible that it would only take a little bit longer than that to create it.