Earlier quoted context omitted.
What exactly does "dehumanization" mean?
It means precisely what it sounds like -- using language which describes the targeted group as something other than (and lesser than) human. The article gives examples where religious groups are described as "rats", "viruses", "filthy animals", or "maggots".
Reductively, calling someone a lion is just as dehumanizing as calling them a shark. In the Bible, people are both complemented and insulted with comparisons to lions. Calling someone a shark is more respectful than calling them a rat, unless you're Chinese in which case I'm not sure what rat means. What exactly is banned here? Twitter gave us only the most obvious examples, leaving the hazy zones open for the endless fighting that I predict is about to start.