18:77 – One whose work has ceased with the dawn of knowledge does not find any opportunity to do or say anything, even though in ordinary people’s eyes they are performing action.
With the dawn of the knowledge, “I am the self” the belief, “I am the doer of action (the body-mind)” is negated. A person with this knowledge knows, “Even when the body-mind acts, I, the self, do not act.”
18:78 – For the wise one who is ever immutable and fearless, where is there darkness, where light? Where, moreover, is there any loss? There is nothing whatsoever.
The wise one no longer believes in the reality of duality, such as the duality between darkness and light. They know—superficial appearances aside—that all is the immutable and fearless self. For that reason, there’s nothing to lose (or gain). “There is nothing whatsoever” insofar as the universe is just an illusion. But that doesn’t mean there’s no self. It just points out the fact that, unlike the world, the self is “no thing” to be experienced as an object.
18:79 – Where is patience, where is discrimination, and where, even, is fearlessness for the yogi who is impersonal and of indescribable nature?
The “yogi who is impersonal and of indescribable nature” isn’t actually a person but the self, because no person can be impersonal and indescribable. For the self there’s no patience, discrimination or fearlessness because those are merely states of mind that never affect the self.
18:80 – There is no heaven, and there is no hell; there is not even liberation-in-life. In short, nothing exists in light of self-knowledge.
Heaven, hell and the person liberated in this very life are merely figments of the illusory world of duality. Their reality is negated in the light of self-knowledge.
18:81 – The wise one neither longs for gain nor grieves at non-attainment. Their cool mind is verily filled with nectar.
A “cool mind verily filled with nectar” is the mind of one who’s fully assimilated the implications of being the self. They know that in everyday life, there’s nothing that can be truly gained or lost. Does this mean they don’t try to acquire anything or accomplish anything? No. An enlightened person needs food, clothes and medicine and goals just like anyone else. But they go about taking care of their needs—and the needs of those around them—knowing it ultimately doesn’t matter one way or another.