To have jam tomorrow means to have an empty promise that will never be fulfilled,unlike how the person in this context is using big words to tell them it is going to be a bleak future that is making people aware of the reality.
Consider the statement "You use big words to tell them it's going to be a bleak future and they hope.". Based on this "You use big words to tell them it's going to be jam tomorrow and they hope." a true statement? Yes, no, or I don't know?