Ethnic cleansing is an action humans have engaged in since they recognized ethnicity as a concept. Fascism is a political philosophy that comes about as a response to Marx's works. There are thousands of years of ethnic cleansings that have nothing to do with fascism.

Fascism is authoritarian in nature and Israel is not a fascist state. There are no fascist democracies.

What a physicist thinks of a political system has the same value as a political philosopher's take on physics.

Edit: you are entirely misrepresenting what the 1948 NYT letter was about which is a rejection of a singular party in Israel and not the state of Israel. Your link does not have Arendt calling Israel fascist but comparing a specific party to fascists. Why are you misrepresenting the truth here?