the movie is based on a short story about a guy who plays hooky from work by telling his boss his infant daughter died. that's why the tagline is what it is.
shareTaglines: He's about to have a moment of truth.
The "lie" is not the obvious one. He and his wife realize in that moment of truth that the life they are living is the lie and is not best for them or their daughter. Yes, I would consider it to be a black comedy, but also, more. You should read more of T.C. Boyle's short stories to get the feel of his writing. I can recommend "After the Plague", a book of short stories by T.C. Boyle.
Like many short stories, the ending can seem ambiguous and leaves the reader to "fill in the blanks." If a short story was tied up with a neat little bow, it would be more like a children's story than one for adults.