We should be careful about conflating principle and evidence. That looks like an evidential test to help a court decide whether copying had taken place. If you

We should be careful about conflating principle and evidence. That looks like an evidential test to help a court decide whether copying had taken place. If you turn up with what looks like a tweaked copy of Harry Potter claiming you wrote it yourself, no-one's going to believe you -- but if you did somehow manage to prove it on the balance of probabilities (e.g. you've lived your whole life in a sealed metal box and can prove you've never read Harry Potter or something), then you're fine, you've rebutted the evidential presumption.