Lehi here reemphasizes the most important aspect of his discourse: human beings are free to act. The simple existence of opposition provides the opportunity to act, but only our acts have moral significance. Therefore, Adam and Eve were unable to exercise their agency without opposites.
In the story of the garden, there is only one opposite: eating the fruit of the tree of knowledge of good and evil. Everything is permitted but that. Therefore, except for that single prohibition, Adam and Eve have no “opposition” by Lehi’s definition and would therefore be in a state where everything was a compound in one, where there really was no righteousness or happiness because there was no choice among the opposites. Without that tree, using Lehi’s logic, there would have been no purpose in creation. Even its name indicates that without it Eden would have been Lehi’s “compound in one.” The knowledge of good and evil is an absolute prerequisite to being able to choose between good and evil.