it's true that anti-Zionism isn't always antisemitism, but "Zionist" can mean different things to different people, so actual antisemites will often claim to only be anti-Zionist
e.g. "most Jews are Khazars. it's not antisemitic to say that, it's anti-Zionist!"
or "freemasons control the world on behalf of Israel. it's not antisemitic to say that; it's anti-Zionist!"
to make things worse, the left doesn't always do a very good job at calling such people out on antisemitism