Zoroastrianism is the classic example of Good vs. Evil. You have Spenta Mainyu, representing good, in an eternal struggle against Angra Mainyu, representing evil (there are other variations on this, but it basically boils down to good vs. evil). Encountering Zoroastrianism had a profound effect on Judaism, which previously lacked the dualistic themes (God was neither good nor evil). Satan was not previously a major player, simply one of God's servants in charge of tempting and accusing mankind. In the Book of Job, for example, he was essentially a member of the divine court, charged with looking for men's sins and bringing them to God's attention, like a celestial prosecuting attourney. He was unable to act without God's permission. This view was changed after the Jews encountered Zoroastrianism in Babylon. Now it became Satan who incites David to take a census of Israel, where previously it was God who did so. Satan was now an adversary of God as well as mankind.

And this was essentially a watered down version of Zoroastrianism's struggle of good and evil. Thus, Garner uses it as a yardstick for dualistic systems.