Dogen says that we shouldn’t distinguish between practice and enlightenment. Practice isn’t the means by which we attain enlightenment. Practice itself is enlightened activity.
And we can see that Dogen is challenging layers of dualism. If enlightenment is distinct from practice, there must be a person who attains it, and his enlightenment – and his personhood – is distinct from the world.
He is primarily challenging the primary dualism, that of Time and being. It is on this dualism that all the others rest. We are born, we endure, we die. Our lives take place in time. But this is a fundamental alienation from ourselves.
In wholehearted activity time does not exist.