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.