Myth is “already” enlightenment because myth and enlightenment have something in common: the desire to control nature, rooted in a fear of nature and aggression.
In myth and religion, human beings tried to control nature (at least to the extent of sustaining the right amounts sun, rain, and fertility) by propitiating the gods, offering them sacrifices and other signs of devotion. Myth and religion understood nature in personal terms, seeing in forces such as sun, rain, and wind the recognizably human qualities of purpose and desire. Knowledge of reality was acquired by means of inspired mystical experiences, and passed down through the generations as authorized by tradition.
The central principle of the Enlightenment, on the other hand, was the sovereignty of reason. Reason is the highest source of intellectual authority, and its findings trump religion and tradition. Reason establishes the value of religion and tradition, but neither religion nor tradition can evaluate reason. As Immanuel Kant put it in the Critique of Pure Reason (1781):
Our age is, to a preeminent degree, the age of criticism, and to criticism everything must submit. Religion through its sanctity, and the state through its majesty, may seek to exempt themselves from it. But then they arouse just suspicion against themselves, and cannot claim the sincere respect which reason gives only to that which sustains the test of free and open examination. (Critique of Pure Reason A.xii.)