They cite the 500K number because that was what GM/Toyota was producing when they were making Geo Metros. Keep in mind, Tesla has been able to make far more use of space due to robot placement and the fact that technology has advanced significantly since then. In addition, they were able to reconfigure things as if it were a blank slate-- quite a rare opportunity. Pair this with LEAN manufacturing and continuous improvement and the fact that the product itself is inherently cleaner. Production capacity of 500K is possible-- it just depends if it's probable.
EDIT: Have work experience with manufacturing and efficiency planning