Why does "these aren't the droids you're looking for" translate to 这些不是你要找的机器人?
"这些" (these) is the correct pronoun for objects in plural number including two

Droid is short for android (a mobile robot usually with a human form) 

"Robot" is translated as "机械人/机器人" in Chinese, despite the fact that not all robots look like human (a car assembling machine with only one arm attached to a box is also qualified as robot in English, but to Chinese, that is just a machine - 机械/机器) .  

Android is designed to look and act like human, it should be more specifically classified as "人型机械人"(humanoid robot). But to most Chinese, "人型机械人" and "机械人" belong to the same group (they are all machines) therefore translate both "droid" and "robot" as  "机械人/机器人" is acceptable in Chinese

You are right, by its look, R2-D2 should not be classified as an android. Using 'droid' in the English subtitles is technically wrong. But it is a fictional story in a fictional world. It is possible for characters in this world treat robot and droid as the same thing (Storm troopers might not be all well trained in linguistics). Or the term "droid" to them generally means "a machine capable of human like behavior" -- in human form or not