That’s a question of standard of proof. If you want a criminal conviction, you have to get past reasonable doubt. If you want to satisfy one of the key canons of journalism, in general, you have to be properly sourced.
However, even the standard practice of Journalism allows for an occasional exception of single anonymous sourcing for a story, based on the judgment of the participants. And getting a criminal conviction is not the objective here, so neither is reasonable doubt sufficient to demonstrate falsity.
But calling it fiction is too strong for me. Fiction implies conclusively false, does it not? I just don’t see that here. I see uncertainty, yes, but not conclusive falsehood. And I see no standard, whether legal, journalistic, or logical, that permits an absolute falsification where the facts suggest a reasonable possibility of truth.
What then? Would you have no stories told that did not expose the teller’s identity, even if that exposure involved lethal risk? Was the writing of “Publius” strictly fiction when he authored the Federalist Papers? And was “Deep Throat” only capable of fiction until the day his true identity became known?
No, while anonymity does introduce uncertainty, there are times and circumstances where the risk of uncertainty is outweighed by the benefit of publicizing an important message. The Antwerp mother, howsoever real she may or may not be, has crystallized with her story a painful irony of the cultural insurgency that is engulfing Europe, and that message is worth hearing, even if shrouded in the uncertainty of an anonymity designed to protect life.
Fiction means not true...once the story is properly sourced, then it becomes non-fiction and is considered to be true or based in reality.
=8-)