Keep in mind that the analogy of the mill is only one element in support of Leibnitz's central argument in the Monadology. A crude outline:

   Perception exits. 
   Perception cannot be found in the parts. 
   Therefore there must be something else. 
   To avoid the same problem the something else
   must not be composed of parts.
   Therefore a simple substance [the monad] which 
   perceives must exist.

What the passage from Leibnitz suggests is that the step from drawing an analogy using most sophisticated technology of the day as representative of the brain to the belief that the brain is the technology is not unique to the invention of computers.