I never understood this: the Palestinian children whose family and limbs are torn from them for Israeli sport are also Semites. It seems like the ultimate erasure to claim "antisemitic" for only Jews.

Etymologically you're correct that semites refers to [all] people from middle east and horn of Africa. But the label is from the European perspective, where it was used to refer to Jews. I'm sure that if they'd had a say in it, they would not have referred to themselves as Jews.

But yeah, there is indeed some irony in the term "antisemitism" in the context you describe

European origin jews are not semites btw. Antisemitism is a misnomer, when they refer to antisemitism they refer to Jews only.

There is no such thing as a "semite." It's an archaic racial category that 19th century German race science used since "anti-semite" sounds more scientific than "Jew hater." Consequently, that's why it's applied to Jews rather than a larger pseudoscientific racial group.

(More broadly, "they're semites too" is the "Elon Musk is African American" of I/P discourse. You can recognize extraordinary human tragedy without re-using race science.)