It is a switching regulator. It actually says: Efficiency up to 94%, note the up to part. The actually efficiency of any switching regulator will be a function of the input voltage, output voltage adjusted to and the current draw from the load. So its efficiency number is variable within a range. Most switching regulator datasheet will have graphs showing the efficiency Vs various operation conditions. But in all cases they all run much for efficient then linear regulators.
As far as buying that unit. I would hold off till you know more about it. I would want to know what regulator chip is being used in it as a minimum so I could consult its' datasheet to see more about its' capabilities and limitations. Also sounds like the seller is warning that output voltage above 8vdc the thing will get loud due to inductor noise?
Lefty