How do they calculate the output power?
From the less than complete information provided in the advertisement, and the use of "3000 mW" on the box and on the device itself and elsewhere, it is almost certain that they are referring to the peak transmit power of the WiFi RF output stage. This is liable to be 3 Watts (= 3000 mW) DC in and not RF out, and there is a moderately good chance that actual power  level is not even that high.

They say " ... The high power adapter greatly improves stability. ..." and " ... Great coverage 3000mW ... ".

It is common to market equipment wit stated power ratings that are either above actual or which relate to the power at the peak of a cycle or over a very short transmission period and which do not represent power as it would usually be measured or actual mean power over even quite short periods of time. 

Peak RF power and mean RF power while transmitting and mean RF power can all be substantially different, and it is possible to have peak powers of 3W from a 2.5W DC source with the extra being provided for the very brief periods required by capacitors. 



If I was building a Quad-Copter I'd want the performance of my coms link to be very well known and to be based on a reasonably complete set of relevant data. This unit cites  "3000 mW" in a number of places but does not mention transmit power in the specifications and makes no mention of receiver sensitivity. About the only concessions to formal specifications are a statement of maximum VSWR and claimed antenna gain. 

For this utterly indispensable part of a quad-copter system I'd buy something else which had more complete specifications and which was produced and backed by a reputable manufacturer. 

One possible precaution against uncertain link performance would be a "fly back towards launch position on loss of signal" mode, with GPS, inertial and other relevant data input - but that's a whole new topic.