Basically it’s very similar to each other. As mentioned in the reading, design fiction is more like predicting the future whereas science fiction speculates about the future as a way of analyzing the present. Design fiction is basically using prototypes to reveal about the future. Science fiction too, but some are quite impossible to be made. Science fiction authors are aware of that as well. For instance, the holograms we see in Star Wars. However, design fiction is to be taken seriously as it creates the future. Bad design, bad future.
Here’s a very good read on the differences: