Should a pre-flight inspection spot snakes on a plane?
Given the location of the snake from the photos/video in the article and the height of the wing/engine assembly (as shown below) I think it's entirely possible to miss this critter in preflight.  A visual inspection of the control surfaces is certainly a part of the preflight, but unlike on a small aircraft (say a Piper Cherokee or Cessna 172) you will probably not be getting "up close and personal" with the control surfaces - you would need a ladder to to so, and the sort of in-depth examinations that would require getting up on that ladder would probably normally be performed by maintenance personnel. 

A pilot doing a preflight might get up on a ladder if they noticed something amiss (say a snake obviously hanging from the wing), but on a simple pre-filght visual inspection from the ground this unfortunate critter could easily be missed - particularly if it was curled up tightly in a small space as snakes are wont to do, and didn't have its body hanging out where it could be seen.