But we know that Helen Thomas is not anti-Semitic because, after all, she told us so. She’s just...under-appreciated. </sarcasm>
The word Semite derives from the Biblical descendants of Noah's son, Shem, who were identified in the Bible with Arabs. The group of languages related to Arabic, including Hebrew, came to be called Semitic during the 19th Century. When eruptions of Jew-hatred surfaced in the west in the 19th Century, (the malady is not confined to any particular century) the only identifiable population of Semites living in the west were Jews, so the term antisemitism is understood to mean Jew Hatred.
Jew-hatred really got the bit in its teeth with the Third Reich and its manifestation there is invariably referred to as antisemitism. Since the Nazis made common cause with Arabs when it served their purposes, Nazi antisemitism cannot by any stretch be construed to extend to Arabs.