It is proof that she wrote a book.
To present an analogy about Susan Fowler with "if a journalist with no experience..." is to present an analogy that is factually incorrect regarding her experience.
If a person created an open-source project on nights and weekends that received industry-wide acclaim, regardless of day job, he/she would most certainly be getting job offers.
They are different jobs. You can write a book without doing any investigative reporting, but it would be hard for a journalist to do so. Siraj Raval might be a published author, but he still has next to no experience being a journalist.
Here's the link in case anyone is interested:
https://www.amazon.co.uk/Production-Ready-Microservices-Stan...
So I think writing a topical piece does make you a journalist, though perhaps not one qualified for prestigious awards.