While seemingly quite similar, author platforms and book funnels are two completely different entities. The shortest summary is an author platform is a community and a book funnel is a sales tool.
My article “The Importance of an Author Platform” goes into the specifics of an author platform but, generally speaking, an author platform is about creating a community for your fans to experience you and your book(s). It’s not just a website but includes social media profiles, a YouTube/Vimeo channel, and more. An author platform can take quite awhile to grow in both audience and content. An author platform nearly becomes a living breathing entity if done well.
A book funnel is the exact opposite; the audience isn’t grown, it’s driven … driven by advertising. The specifics of a book funnel can be found in my article “What is a Book Funnel?” In summary, a book funnel is a sales funnel with a book as the centerpiece. It’s all about the book … and, of course, all of the related products and services. Typically, a book funnel present the book as bait, a free plus shipping offer, that leads into a follow up product which leads to a follow up product. The sales process goes something like this: book >> companion workbook >> webinar >> online course.
Another way to look at it is that in an author platform you speak WITH your audience, with a book funnel you speak AT your audience.