You ask a great question. They quoted him and then stated that it was “not a physical threat” in order to clear him, despite, no kidding, 15+ witnesses to the contrary.
Some how I doubt that your superiors chose not to charge him with making a physical threat to clear him.
More likely it was a CYA moment.
If they charged him with making threats against your life they would then have to explain not taking action earlier (assuming that earlier reports were made documenting his earlier threats).
Such individuals are often ignored or their aberrant behavior dismissed because dealing with them properly is a major hassle and often leads to legal complications.
But if the man does go off and harms someone and the documentation is there showing he was identified as a serious threat they will be in hot water. But the odds are with them and they are for the most part protected by law.