Calling someone names is not an ad-hominem. Making accusations about a person's character can be a valid inference based on their behavior. What makes an ad-hominem is thinking that a person's character (or other attributes) has any bearing on the correctness of their argument.

For example:

"X is a bad person, therefore their argument is invalid" is an ad-hominem. Bad people can still make valid arguments.

"X's argument is both invalid and in bad faith, therefore they are a bad person" is a logical inference.

> Calling someone names is not an ad-hominem

It's not, but it also has no place on HN.

The insult was not directed at the commenter, rather to a group of people that popularized the quote.

It is the difference between "If you believe X then your school must have had brain-dead teachers" and "If you believe X you are brain-dead"

He literally added the context of the quote, without which MLK's views were misrepresented.