There's no doubt that a nice fat dose of fame will help you get your book published: but that's not the only route into publishing.
The majority of books which are published by mainstream, commercial publishers still come from people who are relative unknowns. While I'll agree that it can be difficult to get your book published, the root of that difficulty lies in writing a good enough book, and not in circumnavigating the complexities of the publishing industry.
You don't have to be an established author to get published, either. If you follow that particular myth to its logical conclusion, the fallacy is easy to see: even JK Rowling was an unknown once.