While the “challenge” is incredibly easy to finish, I found it interesting that vehicles, or Ghosts at the very least, counted as an enemy player as well. I was just on the Metropolis mission on Halo 2, and by the time I had got to the Wraith in the middle of the bridge the challenge had been completed. Every time I hit a Ghost with a shot, killing the driver but leaving the Ghost strangely intact, my progress went up by 2.
Most likely because vehicles and AI are treated very similarly in the engine. During Reach’s development Bungie described how they upped the count of onscreen AI/Vehicles from 20 (or so) to 40. If you play with scoring almost all vehicle kills will grant a minimum of a double kill.