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.

I think as a general statement it's true, however the causal reason for that is not because they're heat pumps, it's because of the market for heat pumps is different.

If you poll the current installed base for HVAC, I suspect you'd find that the (vast) majority of systems are minimum SEER rated (i.e., least efficient) systems, so SEER 13/14 (just became SEER 14 this year).

In comparison, if you're already paying the premium for the heat pump system, likely you also pay an additional premium for a higher SEER unit. Also, what I've seen when doing some online shopping that most heat pump systems marketed seem to be higher SEER. Likely, because the cost associated with additional heat pump stuff drives into a higher end market where you 'might as well' make them higher SEER as well (with inverter compressors, variable fan speeds, etc).

The bigger reason is the japanese market demands efficiency. Buyers compare running costs.

Thus Japan acting as a breeding ground for cheaper, replaceable, mini splits. The American manufacturers sold into the HVAC specialist market which was efficiency insensitive but cost sensitive. Thus a bi-partite market.

The only reason mini splits have not bankrupted the American manufacturers yet is many Americans still rely on expensive HVAC specialist to do installs. Thus mini splits extreme cost advantage is buried by expensive installs.