There's no such thing as a billionaire athlete. There are a handful of athletes who went on to become billionaires through their business dealings, like Michael Jordan, or Ion Țiriac or LeBron James. The arguments are exactly the same for them as for any other billionaire - that their businesses are either not really worth as much as the numbers suggest, or that their business engaged in various problematic practices.

For authors, the arguments are again quite simple - billionaire authors or directors don't become billionaires by selling books or movies, they become billionaires from much larger businesses that employ way more people who were collectively much more critical to the success of said businesses than the authors themselves. JK Rowling didn't become a billionaire until the movie franchise and the toy mega production began. If the movies hadn't happened, or the toys hadn't happened, she would have just been an extremely rich multimillionaire.