Tehilim 9 begins like so: לַמְנַצֵּחַ עַל־מוּת לַבֵּן מִזְמוֹר לְדָוִד׃ It contains the unfamiliar words עַל־מוּת לַבֵּן in the opening. What does this phrase mean?

Israel Shahak in his book Jewish History, Jewish Religion: the Weight of Three Thousand Years claims that the majority of Orthodox Jews believe the following quotation. He does not give sources for any of these claims. Is this actually true? Is this only believed by Hasidic Jews? Is this what...