Yeah, Ashely Biden is totally going to confirm that she wrote a diary that claims some inappropriate behavior involving her father as a child.
No, the government or her hasn't vouched specifically for that document, and I don't know why you would expect them to. There's an ongoing criminal case involving it, too, so there's probably legal reasons as well as political ones for them to keep mum about its contents.
But there's more reason to believe it's legit than to believe otherwise. We know what the paper trail for it is. Here's Politico saying it was sold to Project Veritas:
These are the first charges in a federal probe of how the journal of the president's daughter was sold to the conservative Project Veritas outlet.
www.politico.com
From Project Veritas, it went to the National File:
The publisher of the conservative outlet that ran Ashley Biden’s diary says their source was Project Veritas.
theintercept.com
Due to the criminal charges, it's basically been admitted that that's the diary, in a round about way.