Einstein is known to have supported the Labor Zionism movement, and to have opposed groups such as Irgun (considered to be terrorists by many).
The letter doesn't even have the word Zionist in it
Edit: From Wikipedia:
When Israeli President Chaim Weizmann died in 1952, Einstein was asked to be Israel's second president, but he declined, stating that he had "neither the natural ability nor the experience to deal with human beings." He wrote: "I am deeply moved by the offer from our State of Israel, and at once saddened and ashamed that I cannot accept it."
Pretty sure he was a Zionist