Maybe not, but it certainly would "excuse" the prosecution from using the statement to prove callunia. So, we would be left with . . . no case.
You have strange values regarding your doubts and certainties.
The calunnia is a behaviour which the courts found to be "incontrovertible" and independent from how she was treated by the police.
The courts (Bruno/Marasca) also pointed out that her oral statement was "spontaneous" and the context where she released it was "institutionally immune" from possible pressures. The defence never claimed that an interrogation by Mignini ever took place - and a failure to make that claim, if it were true, would lead the lawyers to be disbarred.
When Knox had an opportunity to clarify about her statements and false accusations, she refused to respond questions, she did so twice, on Nov. 8. and on Dec. 17.
Knox also submitted hand written statements where she places further false evidence and she "stands by" her previous statements.
All of those documents, including judicial findings, are usable, even without spontaneous statement.
But the spontaneous statement anyway was not released by Amanda Knox as she was a suspect, because we are talking about a calunnia case, that was still to be committed at the time. She was not a suspect of calunnia. Her statements are the crime itself, its a behaviour, an event first of all.
On the other hand, prosecution would need no copy of these statements in order to prosecute her for calunnia, because the simple testimonies of Anna Donnino and people present would be sufficient to convict her.