Well you sound like a viewer who judges a movie based on how closely it adheres to real life logic and facts, and no judgement here since those are your personal standards, but there are some of us who dont really care about that stuff since we know that movies arent real life.

Suspension of disbelief is a thing because fiction is under no obligation to be realistic or factual. I dont watch a movie like Split to educate myself on mental health disorders, I watch it for the horror and suspense and that dictates the metric I use to judge the movie. I judge it based on how much it keeps me on the edge of my seat worried about whether the teenage girls will survive and in that case I would say the movie was quite good.

Point is, yes the movie is wildly inaccurate with its portrayal of DID, but I doubt any wise individual seeking information on the disorder would refer to a horror suspense film as a primary source and if a lot of people do so then that feels like a failing in media literacy