They don’t now, but I remember in the 70s and early 80s when they were used interchangeably. There has been a lot of effective debunking, and now most people understand the difference.
Personally, I think it’s an iatrogenic disorder most of the time, but those people really manifest personalities, and hav black-out episodes.
However, I have a theory, which is just my lay theory, and IANA psychiatrist, neurologist, or psychologist, but I have dealt with lots of disabled people, and there is a type of seizure that was only recently known to exist, called a “focal seizure,” where a person can remain conscious and function with a portion of their brain, while another part is experiencing a seizure. The person is not usually capable of making long-term memories during a focal seizure, and experiences them as black-outs, even though other people might insist they were conscious and talking.
I knew one guy who had focal seizures that lasted as long as six hours (extremely unusual), and during them, needed to be looked out for, but was still capable of using the bathroom, dressing himself, and getting food that did not require preparation. It looked like he had reverted to a child-like state, and I think a therapist who was both predisposed to see DID, and unaware of focal seizures might have thought he had switched to another personality.
Anyway, given that DID patients always seem to have child personalities, and iatrogenic psychiatric problems are very real, I wonder if focal seizures and over-eager therapists explain most cases of DID. Or, at least most early cases. I would not be shocked to learn that many people post-Sybil were faking. And I feel sorry for them. They were probably abused, and neglected emotionally as children, and wanted a nurturing relationship with a therapist like the one in the movie.