The words ‘eateth in darkness’ tells us the verse is a figure: namely, that he gets a lot of sorrow out of being rich. Even though he may be rich, his riches have failed to deliver what they promised when he first set out to obtain them.
The difference between living life in darkness, and living life in the light of understanding is vast. The tragedy is that we can pass right through this life without knowing why we are here, or what is the chief purpose of life. The atheist works overtime to justify his worldview, but he treats his soul in an unnatural way, and it will always secretly rebel against such treatment. We cannot fully come to terms with meaninglessness and with death.