I agree it's ridiculous. It's called fiction - I don't see it as cultural appropriation, just telling someone else's story. A made-up someone else. Is the reverse true? Should I only read stories about white, middle-class women with children and a professional qualification? Are we to have books where all the characters are the same to ensure that nobody's offended by someone telling 'their' story?
It's also somewhat prescriptive about what people can write about. Maybe, if the (hypothetical, for the point of the article) queer Indigenous man was a writer he wanted to write detective novels, or historical romances. Is he to be told that he is only allowed to write about what he knows as well?
Dave Eggers - a white, now rich, middle-aged American wrote a brilliant story called 'What is the What' about a Sudanese refugee - by the Guardian article's argument books like that should never have been written. Even Jane Austen, who clearly write about what she knew, shouldn't have been published as she's appropriated motherhood which she had never experienced.
Possibly I'm irrationally annoyed because author's writing in the piece was just so, so terrible - and technically incorrect - the beating of the heels on the carpet would be half the pace of a heart infused with adrenaline.