This is not true in the slightest. In fact, most published authors are being pushed to develop a social media presence even if they already have an agent and a successful publishing history, and a lot of debut authors in the last few years started on social media through Youtube, Instagram, AO3, etc. Publishing houses want to know that they already have an audience to sell to which is almost just as important as the book content itself. I think Ruby's challenge is she wants to debut with a retelling and the audience she currently has is mostly composed of people who joined for studytube content and of those people there's a vast amount who are older now and not a fan of her or her writing. Retellings are already kind of polarizing because most of the audience already knows what the story is about and have already decided whether or not they would be interested before the book is even published. The fact that she hasn't done a good job pivoting her Youtube channel and that she isn't the strongest writer means that she has this thread for example to contend with when it comes to selling the book.
I might need to take a break.