Is it ethical to diagnose Donald Trump with a personality disorder if you’ve never treated him?

Back in 2017, a book was published called The Dangerous Case of Donald Trump: 37 Psychiatrists and Mental Health Experts Assess a President.
Each author separately postulated that Mr Trump had a narcissistic personality disorder and as such was unfit to hold office.
The features of the disorder listed in DSM-V are as follows:
Read more: Psychology: Why Trump can’t admit defeat