Just because somebody worships only one God doesn't necessary mean he doesn't believe other gods exist.
The early Hebrews believed that YHWH was the tribal God of Israel, that He was greater than the gods of other nations.
A very differnt perspcetive from what came later - the realization that only YHWH existed and there never were "other" gods at all.
Look at the ten commandments "I am the Lord thy God, thou shalt have no strange gods before Me", instead of "I am the Lord thy God, the only God which exists."
Sure, there were idols, but only one God (capital "G"), that is there are spirits but only one God, one Creator, one Lord who reigns over EVERYTHING else.
David never believed there was competition for this Supreme place. "Hasten, O God, to save me" is a constant refrain in the Psalms. Not "Hasten, O Gods" but singular...."God".
You will not find plural worship advocated by Jewish leaders in books like the Psalms or Job.
If a Jew does slip into idolatry, he is castigated. Even the high priest, Aaron, was castigated for this.