I think the point was that any comment or remark that you'd try to police with that statement can be flipped around towards you.
First of all: who is to decide what's dehumanizing, or even hate speech?
It's also possible to make a comment that is read by one part as hate speech, but it was never intended as such. The quick attack made on the poster that made the: "I feel that this comment dehumanized me." makes it seem like someone is offended by that very comment. If I, by accident, make comment, that someone decides is hate speech and try to have it removed, I would certainly feel dehumanized. Having the meaning of my words ripped apart and banned by a system that decided that I must hate someone, that's hurtful.
Hate speech isn't what worries me though, it's the notion that me speech somehow needs policing. First we ban the obvious stuff, not attacking people on the basis of gender or race, we can all agree on that. Next it's a ban on hateful speech against religion... but we don't all agree on that one, because some religions need to be criticised. Anyway screw the anti-religion people, who next? Those who go against the government, how about those opposing certain parties. In the end I'm kicked of the Internet for saying something negative about Disney.
How is your life any harder to live because the parent comment was made? How has any positive human quality you innately have been refuted by the comment?
There is a clear definition of hate speech. It's not as amorphous as you need it to be in order to protect your ego.
There is. Many people use it. Have you ever gone out of your way to examine it? Or do you just infer it based on your internal beliefs and limited experiences?
There's no need to assume that anyone that questions it hasn't had experiences regarding it. Just the fact that there exists in your response any implication that a lack of experience may change someone's definition of "hate speech" makes it amorphous. There may be a definition, but it being "clear" is questionable. Obviously the answer is to leave it to the courts, but then you have to consider the definitions of "attack" or "intimidate" or "discomfort" and if any amount of these is "violence" and to what extent is it punishable.
(see the answer?)