It's from that last line that I'd like to suggest that while it's perhaps accurate to apply "professional writer" only to those who make a living as writers, the material realities of the writing life
make such a determination numerically meaningless. So few writers actually make a living as writers, and of those that do make a living as such, most of them do so via a variety of writing avenues. A midlist author of science fiction novels, for example, may fill in the enormous gaps from fiction publications with freelance work (essays, editing, etc.). The number of authors who actually get to live off a single form of writing (Stephen King, for example, or Neil Gaiman...) comprises such a small number of all published writers out there that using "professional writer" on them alone wouldn't really tell us anything other than "these are the authors who sell enough books to pay a mortgage." Since a great deal of non-writer folks likewise wouldn't fall within the domain of a "professional" based on how well they do in a given field, I just don't see why the term provides any use value if we apply so selectively.
And that's perhaps the big problem here. What the hell is a professional writer anyway? Would Harper Lee count as a professional writer? She only wrote one book: To Kill a Mockingbird. But it sells so many copies every year that I suspect she could live quite comfortably off the various royalties and rights purchases associated with it. Is she a professional writer? By the standard of financial value: yes. By any other standard of professionalism? Nope. Most uses of the term professional apply to those who actually participate in the production of a "thing." A doctor who has a practice or works at a hospital is a professional. A practicing lawyer is a professional. An author who sells one book and nothing else? Well...
I suppose all of this is essentially a reflection about the state of the field of authorship. In other fields, one can become a professional by "doing," but in the world of writing, I'm not sure there's an easy measurement for "professional" and "not." Harper Lee is probably a professional writer, but the standards by which her professionalism would be measured wouldn't apply to someone like, say, Tobias S. Buckell, who still splits his salary between fiction sales and freelance work (I'm not sure how true that is today, though; he used to do these in-depth analyses of his yearly salary, but he's been quite busy lately). In Hester's assessment, the former is instantly a professional writer; the latter is not. Why? What makes the distinction here? Money can't be the only valuable distinction between the two. There have to be other factors, too; otherwise, what's the point of calling anyone a professional writer if all you need to do to become one is publish one book and sell millions of copies?