What Is A Typical Computer Science Ph.D. Interview Like?
There really is no &quot;typical&quot; interview protocol for CS PhD positions. The kinds of questions asked will mostly depend on the qualities and skills that the PI deems relevant for the position. If the work requires specialized technical knowledge, it's possible that the PI will try to test this knowledge, and programming tasks are a valid way of doing that.
In your case, one could argue that the PI slightly miscommunicated the contents of the interview, since a programming task is arguably not a programming question.