
Semitic peoples include Arabs and most inhabitants of North Africa, but the word “antisemitism” is used to refer to bigotry against Jews and not against Semitic peoples as a whole. The word refers to Jews it just has confusing etymology. There should be a better word but that is what we are stuck with.
The word “nice” originates from “nescius,” meaning ignorant or stupid. But we don’t call someone nice as an insult. Just because the etymology indicates a word should mean something else doesn’t actually change the meaning. The meaning is how it is used. Anti-Semitic *should* mean bigotry against all Semitic peoples. But that’s not the actual definition.