If you go to HR remember to present it as a company problem, not yours.  
What your boss basically done is to give anyone fired i9n the future a ground to sue the company for unlawful termination. Anyone in that room have 7 witnesses that he have threaten with firing anyone as a repercussion for his [boss] bad review not based on performance of each employee.  
IMHO this is how it should be taken to HR. Not to mention that he defied the logic behind anonymous evaluation. If he will held his position after this evaluation make no sense because everyone will be afraid of being fired if they won't give 5/5.