Biden as a Senator or a VP has *zero* authority to retain documents as a private citizen, which he has done for 40 years now.

As did Donald Trump. Technically, all government documents are the property of the government. When you leave a job, your work stuff should stay at work - only your personal effects should stay with you.

But that's a very different question than what constitutes a crime to retain. The statute is pretty clear - it is criminal to retain national defense documents when then the government asks for them back. It's not a crime to continue to possess such documents before they ask for them back. The crime lies in the refusal to return them, not in the possession or retention.

Recall that the FBI recommended felony charges for David Petraeus for doing what amounted to the exact same thing (retaining classified information and showing it to his biographer). Petraeus would go on to plead guilty of of unauthorized removal and retention of classified information.

Yep - because there was plenty of evidence that Petraeus knew that the information he was providing was classified information at the time he gave it, especially since the documents he shared with Broadwell had classified markings.

There's no indication that Biden let his ghostwriter see any classified documents - instead, the accusation is that he read aloud from his personal notebooks (which would not have been reviewed or marked classified). Which makes it very hard to prove mens rea, since those notebooks would have never have been formally classified. Classified information is classified whether it's marked or not, but without the markings you don't have evidence that any particular person actually knew that a piece of information was classified or not.