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It's a futile exercise, since if one defines science fiction as a subgenre of fantasy that uses science and technology to provide an air of verisimilitude, then no matter how hard you try, the slipstream is still sci fi. Now, you can argue that your book may be science fiction by definition and not by description, but that's another thing entirely. Science fiction is a genre, which means that it is inward looking and seeks to provoke a reaction rather than a response in the reader. It has many conventions, jargon and so on that the reader expects and it is in its essence a mixture of the Gothic and adventure fiction. However, Atwood (whose intellectual powers I've always found wanting) is wrong to claim that science fiction requires explanations and since she doesn't explain, she isn't doing science fiction. There are many examples of solidly science fiction stories that don't provide neat explanations of how the scenario came about. Even when an explanation is provided, it is often a way of establishing the credibility of the plot. Having read Atwood's novels, it would be fairer to say what she does isn't "slipstream", it's sloppiness wrapped into pretension.
At the end of the day, she is a mediocre sci fi writer who tries to gain the status of literature by flying under false colours. Sorry, Miss Atwood, you are no Orwell.