If anyone could coup-de crace her, than the CR wouldn`t be right, would it?
That probably is the main difference between Strength/Dex 0 and Strength/Dex - . The first would be helpless.
I can imagine some reasons why she does not has no Scores, now when I see it - if she had some, but only very weak (maybe 2 like above mentioned), and someone was able to deal Strength or Dexterity Damage, she would be very fast very helpless (paralyzed), which is not what you want the most dangerous Formian Creature to be...
Mustrum Ridcully