If power is transmitted using near-field methods such as the direct inductive coupling used in RFID, most cell phone wireless chargers, and electric toothbrush chargers then the power transmission is coupled directly to the recipient device, and if the device is removed and the transmitter is still running then very little power is radiated away. This is potentially quite efficient. If you use far-field transmission then power is being radiated away whether a device is there to receive it or not. In order for it to be efficient you need high antenna gains to focus the power into a narrow beam the size of your receiver antenna aperture and beamforming to actively steer it. Otherwise the antenna is just spraying a broad beam of power in the general direction of the receiver like your sprinkler analogy.
They say it's using far-field transmission of power:
WattUp's RF transmission, which operates at 5.7MHz and 5.8MHz, is referred to in the industry as "far-field" wireless charging. Energous is not the first company to come up with the idea.
At such low frequencies it would be very hard for them to implement high gain antennas and beam forming. Also at such low frequencies I wonder how it can really be far field since the receiver will be well within a quarter wavelength of the transmitter.
More information is needed to fully answer the question, but this sounds like a convenience feature rather than an efficient one.