Ah, I see where your misunderstanding came from.
Yes, NHSTA publicly rates cars 1-5 stars in a variety of areas, as well as overall.
Internally, they have a ton of specific, quantifiable scores on various tests. This data they share with the manufacturers, but not the public.
However, there are a few data points that are public.
One such example is the rollover rating.
Among all of the vehicles that scored 5 stars in every category (about 2% as I recall), the Model S scored the lowest rollover chance (by a long shot) of any.
So yes, by NHSTA's own public test scores, the Model S is the safest car ever tested.