I think this question comes up for authors on the internet somewhat frequently: do I need to have a brand of self, or a platform, in order to sell a book? They see book deals announced from popular TikTok accounts, they see the writer with the popular Instagram account get a splashy sale.
For nonfiction, I would argue that this is pretty obviously ‘yes’ and has been ‘yes’ for awhile. A large nonfiction book deal requires the author to be an expert in their field, or for the author to be a celebrity, or to have a ‘built-in audience’ of some sort that the publisher believes will follow the expert, the celebrity, the brand from the medium where they are already popular to a book. (The many celebrity books that tank, or the crash of the bubble of books based on popular TEDx talks, shows that this isn’t necessarily true: just because a person is popular in one medium doesn’t mean that fans will follow them to a book. But without any other tangible evidence of how this writer will convince bookbuyers to buy their book above any others, it’s still what publishers use as a metric.)