The prototype doesn't have to meet any standards, it's just a prototype. There's a cutoff for how many vehicles can be manufactured without crash testing, etc. but I don't recall it off the top of my head. Something like 300, maybe? (Quick Google says it's currently 325 cars).
As far as the Prius getting a "free pass", I don't know anything about that. Here's the NHTSA results from the first year it was available in the US: https://www.nhtsa.gov/vehicle/2001/T...IUS/4%252520DR second year: https://www.nhtsa.gov/vehicle/2002/T...atings-frontal etc. What pass did it get?