one common form of antisemitism is associating Jewish people with inauthenticity. So saying Jews are not Jews, or is a form of antisemitism. It's bigotry. it's a form of targeted hate.
When Fred Guttenberg calls Bernie Sanders an "antisemite" and dismisses Brad Lander's Jewishness, he is defining identity through an association with Israel's right-wing government. It's similar to how MAGA often defines true "whiteness" as allegiance to far-right politics.