Maybe because she is directly involved herself in the crime?

Besides this, jurisprudence of calunnia is about
malice, rather than about knowledge. What matters is that you
think or you
assume the person is innocent, not that you have a proof in front of you.
No. First, must be said there is absolutely zero evidence the police pressured Knox to name Lumumba, despite the innocentisti wild assertions of the contrary, and btw there is also an admission onf the part of Knox that she made the name first.
Second, even in the event the police pressured Knox to name Lumumba, that wouldn't be a sufficient justification; Knox would need to give some evidence that for some reason she was
completely mentally incapable in that moment therefore she had no faculty to know or will and could not whithstand external pressure.
Third, actually Knox repeated the action of giving false testimony against Lumumba several times and over a long period; the police interrogation had ended at 01:45. But she made a new accusation without being interrogated, with no police questions, in a statement that was put on record at 05:45 before a Magistrate. Then she wrote down further two hand written memoirs in which she presents again a false testimony that places evidence against Lumumba. And finally, she chose to keep her position and to not answer any question before the Investigating judge on Nov 8. and she kept silence about Lumumba's innocence (or wrote further ambiguous statements) for weeks.