Right you are, in the case of real-life scenarios.  This comic was referencing the "Zombieland" take on the rule which goes something more like, "If you hit them and they go down, hit them one more time in the head just to be safe."  Since zombies tend to hop back up unless the brain is destroyed, it's a pretty good rule to live by.  Thanks for the IRL lesson, though!  Every day's a school day.