The claim is false: The NHTSA rates cars on a 5-point scale, and gave the Tesla S a 5-point rating, the highest they could get. This rating is based on several sub-ratings, where the Tesla also got 5-point ratings, in all categories.
Tesla is basically trying to claim for marketing purposes the fact that they got 5-point ratings in all of the subcategories (which isn't necessary for a 5-point overall rating, and in fact is extremely unusual, if not unique) means that they got 'better than a normal 5-point rating'. Which, ok, they did, but the rating only goes to five points. They can't create a new rating scale just for themselves.