Merely halving the time for mounting and assuming a 20km/h travel speed without any stops (bearing in mind this is a combat zone) seems a bit arbitrary. In any case, the radio calls seem to affirm that the ground element was in visual range of the people before the gunships even opened fire and Namir's photo, if it was indeed taken at the scene as we assume, shows the ground force as being pretty close, perhaps no more than 100-300 m away.