When they have a dispute, they come to me and I decide between one person and another, and I make them know the statutes of God and his laws.
Exodus 18:16 ESV

The minute statutes that Moses gave to the people are the statutes of "God and His laws", not the statutes of Moses. God’s Law existed before the Ten Commandments were given at Sinai or else Moses had nothing to explain to the people.

This is because Moses knew God’s character, which dictates his Law. If you know God, you also know his Law. If you don’t know God’s Law, then you don’t know his character either.