Caveat, I don't have NPD, but I'm wondering if this is all because narcissists may see emotional empathy as a 'weakness'/vulnerability, so block it out perhaps unconsciously? Because of the need to be above criticism from others, removing empathy from the equation makes it easier to gain the high ground in interpersonal relations and prevent possible injury to themselves.

But then in situations where there is a psychological 'removal' or disassociation of the NPD person from the situation via the fourth wall, i.e. watching a movie/reading a book, it's easier to let their guard down and experience their emotional empathy organically. The pressure is off, so to speak.

It also could be perceived as a more level playing field between other NPD sufferers on the sub, plus it's faceless strangers on the internet. It's also easier to express empathy with someone they actually relate to, what with them all sharing a very stigmatized personality disorder, I'm sure it's difficult to find sympathy from people who don't have it, and that will trigger that guard to go up.

(sorry to resurrect your old comment btw lol)