What does Paul mean when he says that a mediator does not represent just one party; but God is one?

This verse is difficult to translate and has resulted in numerous different interpretations. In its immediate context, Paul is busy comparing the covenant made with Abraham (Galatians 3:18) to the law that was entrusted to Moses (Galatians 3:19). Given this comparison, he is most likely drawing attention to the fact that God spoke directly to Abraham (Genesis 12:1–20, Genesis 15:1–21), while at Sinai the angels were also involved.¹ Implicit in his argument is the idea that direct revelation is better than mediated revelation, and thus the Abrahamic covenant is superior to the one made with Moses.²