If you go to HR, remember to present it as a company problem, not yours.
What your boss has basically done is to give anyone fired in the future grounds to sue the company for unlawful termination. Anyone in that room has seven7 witnesses that he has threatened to fire anyone as repercussion for their 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 holds his position after this, evaluations make no sense because everyone will be afraid of being fired if they don't give 5/5.