Equating being against Zionism to being against Judaism means that you are equating Zionism to Judaism, and doing so implies that being Jewish means that you support the continued existence of the State of Israel by default. This means that you are portraying an entire ethnicity and religious group (in this case, Jews) as being ideologically and philosophically homogeneous. So, saying that “Anti Zionism is the same as antisemitism” stereotypes Jews into necessarily supporting Zionist ideology, which in of itself is actually antisemitic. Anti Zionism is the same as antisemitism