What are you talking about?
Scar, without looking, preempted an Alien's sneak attack and cut off its face. Let's remember what we're dealing with here - Aliens are the ridiculously, unfathomably terrifying hybrid of a Ringwraith, a dragon and Spiderman. Put a Ringwraith, a dragon and Spiderman in a blender, and if a lifeform walks out it's an Alien.
Scar played the Alien's own game and won without breaking a sweat.