There's a small, politically motivated subgroup of people (not saying you are one of these people, just that you are buying what they are selling) who want to cause confusion by screwing up a perfectly well known word. You know what antisemitism means when someone uses it though. So do they. They just don't like being called an antisemite, so they try to f up the word.But there is obviously no consenses on what it is if there is people(not just me) saying that arabs are semites too. That Jews aren't the only semites. Even if it used to be used to refer to only jew hatered, that doesen't mean anything if the language evolves. If I told you I was gay you would think that I like men romantically. If I then get mad at you and say "I mean I am happy!" that would be very silly of me.
There's a difference between gay and splitting off semitism from antisemitism. It's that gay is a single word. "Antisemitism" is a single word. It's like complaining about "Irregardless". Yes, technically, you can break down the word to "not without regard" = "with regard", but it really means "without regard".
Language does not work via lego blocks. Homophobia doesn't mean fear of gay people. Nor does Islamophobia.
Terms start from weird places, even hateful ones. It doesn't really matter though, what matters is how it is used.Anti semitism came from Germans they wanted a word so they could dress up their bigotry against Jews and make it sound scientifically "Oh no I don't hate the Jews because they refuse to convert to Christinity. I dislike them because they are a foreign ethnic group not indigenous to Europe. They are from the middle east, I am against having middle easterners in Europe." Before this Jew hate was because of mostly religious dislike, after that it was "scientific" they claimed that semetic people had certain biological traits that put them at odds with Europeans.