JK Rowling was in fact a prole when she started writing Harry Potter. Right up until she started collecting royalties for the IP she was a prole, and when she sold the movie rights for a huge cut she transitioned from petty bourgeois to full blown bourgeois. Even still she is just barely a billionaire, along with Paul McCartney and a handful of other very successful proles-to-start-with. This kind of social mobility is possible
but exceedingly rare. But JK Rowling is bourgeois because of her ability to leverage her labor into class mobility, not because of the nature of that labor. Most writers are proles.