"If the president really is — gulp — insane in the clinical sense ..."

Insanity is a legal term, not a medical term. Milbank tosses around a bunch of unfortunate colloquialisms for literary effect that are offensive and do disservice to those suffering mental illness, their family and friends, and their mental health providers. He should know better.

The legal term refers to someone lacking the understanding of what they are doing or have done, and therefore cannot be considered responsible for their actions. President Trump understands exactly what he is doing even if he may have a flawed or incomplete understanding of the implications and consequences.

Based on his behavior I believe that he certainly has one or more personality disorders and that he has Attention Deficit Disorder.