Calculating battery time giving its capacity and load
Specifications here: http://www.uavfactory.com/product/69


Energy = power x time (Wh)
Duration = Energy / power  hours or seconds



  The battery is said to have 640 Whr capacity
  (actually this is the discharge rate)


No. Capacity is capacity and is correctly stated in Wh = Watt-hour
Capacity = Energy available = Power x time = (here) Watts x hours.


  the load being a 2700 W motor. 


The missing link is that they are claiming a MAXIMUM (implied) motor power of 2700 Watts.

Your overall problem is not appreciating the various units used and how they relate.

Power = instantaneous rate of doing work. Unit = Watts.

Energy = Sum of Power x time products. Unit = Watt.seconds = Joule, or watt hours.

A motor that runs at a POWER level of 100 Watts for one hour (3600 seconds)
uses 3600s x 100W
= 360,000 Watt seconds of energy
= 100 Watt hours.

So if
the available ENERGY is 640 Wh
and the duration = 110 minutes
= 110/60 ~= 1.83... hours

then IF the figures are correct at the power level involved then
MEAN power = Energy/time_operating
= 640 Wh / 1.83h ~= 350 Watts.

The actual Watts will vary across the operating cycle and
the battery capacity will vary with discharge rate
and load is affected by windspeed and air density and flight profile and ...
and some of these vary with time of day and location and ...
so such figures are an approximation.