I'm being a pest, I know, but the legal issue in play that night was not the "voluntary" nature of any admissions, it was the "spontaneous" nature of them.
No less than Giuliano Mignini covers this at length in his 2010 interview with CNN's Drew Griffin. Mignini defended his admitted encouragement for Knox to, his words, keep making spontaneous statements and all he would do would be to record them as if only a notary.
That legal chicanery did not make it past even the Italian Supreme Court, which until March 2015 was favourable to Mignini's wrongful prosecution. The issue of the non-spontaneous statements (made, further, without mandatory legal representation) is something everyone should agree on; regardless of which side they've been on in the past.
I'm being a pest on this, but the difference between voluntary and spontaneous is what's at issue here.