Yeah, but most people cannot take constructive criticism, whether they think of themselves as being professional or not. They might put on a good face but are internally seething that someone dared critique them. It's a difficult line and if you need to give that sort of feedback to someone because you are actively working together, that's one thing. People can be professional, as you say, and can work through it.

But if you are leaving a company, that's even less of a reason for the person you're critiquing to listen to what you are saying (b/c they can just write it off as someone who has sour grapes or was leaving anyway), and even more of an opportunity to just create unnecessary animosity. And that professionalism that becomes a necessity when the person is still a coworker can disappear completely.

Sure, but the GGP comment said they would be giving you a bad reference anyway, while the GP comment (and the article) made the point that sometimes they will only give you a bad reference if you manage to offend them during the exit interview.

Some people are just insecure and always get offended by constructive criticism, some people have poor communication skills and think they are "just being honest and constructive" while actually being very offensive (we probably all know a few coworkers like this) and sometimes the person doing the exit interview has just had the worst day of their lives. Bland platitudes in the exit interview will almost never hurt but candid feedback can definitely go sideways very quick. Game theory says it's better to play it safe.

I would agree with this take but there's still a point about why I worded it the way I did.

I thought about wording it differently (e.g., a person will behave professionally until ...) to help illustrate that but the way I chose to word it also helps to illustrate the perceptions of the other individual: it seemed very much that they were professional until their behavior changed.

most “professionals” aren’t professional about this (in my experience)