You're assuming that particular flight was targeted. Think of it like duck hunting: You sit in the blind and wait for a shot.
Go back and read what I posted in #45. Sitting down there and waiting for the right opportunity to shoot down a passenger aircraft is almost the equivalent of sitting in your back yard waiting for a duck to fly overhead on its migratory route at a height of 500 feet. Anyone who intends to shoot a duck is going to plant himself close to a body of water where ducks flying overhead are going to be well within the range of the gun he's using.