I just found a paper in trying to figure this out, but it seems like the author of this study wasn't really looking at it as an autoimmune disease, but a post-viral syndrome like Chronic Fatigue Syndrome (ME/CFS) which is partially immunological, but not autoimmune. ME/CFS has been known about a lot longer than long COVID, and seems to be better (if not yet well) understood.

Reading though a lot of the sites with information on ME/CFS, it makes intuitive sense that long COVID has more in common with it than something like rheumatoid arthritis. I hope that long COVID brings attention to ME/CFS, or in studying similar diseases we're able to learn more about their common causes/treatments, or generally understand both better.