A heat pump is an air conditioner that can run in reverse. Put another way, an air conditioner is a marketing term for a specific kind of heat pump. Your correct that there is no inherit efficiency different. Perhaps what was meant is that they are much more efficient than traditional resistance heating?
I guess to clarify, where I've lived the most common type of AC is a window-mounted unit. IIRC those are generally less efficient than the heat pumps getting installed today, to the point where my family got paid by our utility to replace the window units with them.
