The real question is, "If people gave interview feedback, would they get sued for it?" I think so. Doubly so if the feedback was at odds with the candidate's self-image or if it is written. (It is a lot easier to file a lawsuit based on what was written to you than your memory of what was said.)
More importantly HR departments universally say so. And as long as HR departments say so, managers will follow their advice and not give interview feedback.