It means that no matter what you write, be it a biography, an autobiography, a detective novel, or a conversation on the street, it all becomes fiction as soon as you write it down. ↗
I hate biographies which say, I was called to such and such an office, and he offered me so and so, and I got so and so money. I find that very tedious. The best biographies are written by other people. ↗
I was reading William Shawcross's biography of the Queen Mother, dressed in my witch outfit! And you know what? It was a really good mix; it was a therapeutic mix. ↗