I think this is a difference of degree but not kind. Duke still has tenured faculty. He is clearly not afforded the rights of tenured faculty. For this matter functionally he's an adjunct because that's how they ruled on him.

Precisely this. I find it appalling that an associate professor was not given tenure, because that is normally a tenured position, particularly since he'd been there 20 years

I think he was an Associate Professor of the Practice -- it's not a tenured position, but more a longer-term teaching-type appointment. It generally offers more job security and benefits than an adjunct might receive, but at many universities such a position is not (and was never) on a tenure-track line.