Earlier quoted context omitted.
I've heard this many times before (I think Charles Bukowski used to preach this?), and I have to say I disagree. Famously Douglas Adams had to be forced to write every sentence in the Hitchhiker's Guide. He certainly didn't have a book inside him that had to get out, he had external deadlines and rent to pay. Can you imagine a world without HGTTG?
What happens when you walk up to a publishing agent and tell them you don’t have anything to show them, and they’ll need to force you to write every sentence but assure them it’ll be brilliant? I think you’re also talking about one of Adams’ later books after HHGTG once his publisher was deep in hock for his advance.
They told him to hand over whatever he'd got:
"Douglas happily built his reputation for (at this stage, merely) sailing close to the wind when it came to publishing deadlines by telling interviewers that he had been ordered to literally stop writing and hand his manuscript over to a courier no matter where he had got to…"
— The Frood: The Authorised and Very Official History of Douglas Adams and The Hitchhiker’s Guide to the Galaxy by Jem Roberts"
He was famous for scraping or missing deadlines from the very start, and openly admitted to it:
"All of my life I've been attracted to the idea of being a writer, but like all writers I don't so much like writing as having written. … each time I meant to try to write something, I'd miss the deadline by two weeks."