They did not need to, the Italian constitution says people are innocent until conviction is confirmed by cassation. Therefore Sollecito is and ALWAYS was innocent. Knox is and ALWAYS was innocent of murder, theft, etc. She is presently guilty of callunia.
I think ECHR may well rule that the conviction violated her rights, this will not void her conviction, and I am unsure if the Italian system has a way to void it. Potentially a civil litigant could put a freeze on damages paid from Italy to Knox pending the outcome of a civil action.
Italian procedural law allows for a "revision trial" in the case of miscarriage of justice as provided for by CPP Article 630 and, in particularly relevant to the Amanda Knox calunnia case, by Constitutional Court judgment 113/2011: a convicted person (or a prosecutor) may request a revision trial when it is necessary to reopen the proceedings in order to comply with a final judgment of the European Court of Human Rights.