Design fictions are closely related to the idea of diegetic-prototypes. There were initially adopted as a pedagogical practice in design schools, to help students be more creative.
I’m fully located in the ‘all design is fiction’ camp, so I’m not a big fan of nomenclature and niche land grabs. Design as a practice never exists in the here and now.
It treats fiction as a kind of testing ground for technologies. Because all technology, and by extension, all design is ideological. It is path-dependent. It also helps to normalize ideas (manufactured-normalcy) and increase adoption through storytelling. In solutions to problems in an increasingly wicked-world, it allows designers and technologists to model complex interdependent issues as simpler to understand stories.