The idea has been attributed to Carlos Fuentes, who claimed writing was rebellion against God. Writers do exercise godlike creativity—not as rebels but as devotees. (Fuentes is projecting, perhaps?) Creativity, by definition, is an act of love.
A text book by James C. Raymond, Writing (is an unnatural act), asserts some alternative interpretation, but not having read the book, I can’t say what that is.
I’m assuming the general idea is that we shouldn’t be too surprised the first time we try our hands at writing and fail miserably. In that case writing is an unnatural act. Because anybody can (and lots do) scribble down words and claim they are writers, but that doesn’t make them such. The term “writer” implies a product of a certain quality, like “carpenter,” or “guitarist.”
Writing “well,” especially writing long (for example: novels or biographies), isn’t natural in the sense that breathing or walking is because:
1) all writers struggle to one degree or another against an internal editor that is always niggling them to reread, rewrite, rearrange;
2) it’s impossible for writers to keep all the elements of a story in view simultaneously, hence the need for all sorts of organizing schemes and visual aids;
3) it’s virtually impossible for writers to be proficient at all the technical skills required to write well until they’ve written a lot—that is, before they've spent a vast amount of time learning from their mistakes and practicing how to fix them; and
4) good writing has an undefinable something, a "je-ne-sais-quoi"—and how the heck do you learn that?
Writing is an art. Just like playing piano or drawing portraits or carving cameos.
Art isn’t natural. (Neither is cooking, but that doesn’t mean we should stop eating.) It takes long effort, sometimes a lifetime, to get really good.
And that’s the whole point, isn’t it?