“What does he mean that there are no donut trees in the Garden of Eden?”

Simply, it means just that. There are no donut trees in the Garden of Eden, just as there will be no bourbon-bushes in heaven, nor jewelry-shrubs in the Resurrection. It points to the fact that God created a world with a potential for further creation. It points further to the fact that when God created Adam and Eve, He created them in His Image, and that a part of that Image is being a little creator —with a lower case “c”— like Him.