Neither of them "founded a start-up" while living below the poverty line. Rowling wrote a book in her spare time that eventually got popular. She wasn't putting any money or asking of anyone else's money to fund her book.
Oprah followed the more usual path of "get a job, get promoted, rinse repeat" until she was a household name before she started her own company. Per the article you yourself linked, she was co-anchoring the local evening news at 19. That isn't the same thing as taking a risk on a start-up.
Writing a book while on welfare is certainly a startup. She didn't self-publish the book. The publisher provided the funding to edit, print, market, and distribute the book (which is not trivial money). Rowling pitched the book to several publishers before one decided to fund it.
Oprah bootstrapped herself out of poverty to become a media billionaire. You might say she took the long way by routing through various jobs to get the money, but she still came from poverty, founded companies, and became a billionaire.
Oprah followed the more usual path of "get a job, get promoted, rinse repeat" until she was a household name before she started her own company. Per the article you yourself linked, she was co-anchoring the local evening news at 19. That isn't the same thing as taking a risk on a start-up.