(it's not actually even the same symptom at all but HYPOTHETICALLY) why is experiencing one symptom of a disorder without experiencing the others (so not having the disorder) ableist ONLY when it's a CDD? it's fairly widely acknowledged that you can have, for example, one experience that could be considered a symptom of autism while being allistic, why is this not the case for this specific group?
While yes, it's entirely possible to have symptoms of disorders without having the actual disorder, and this even applies to dissociative disorders, it specifically doesn't apply to the symptom of multiplicity. Many people claim to experience multiplicity without having any other symptoms of CDDs, but as far as science currently goes, that's impossible; those claims are mistaken in one way or another.
Using research such as the Theory of Structural Dissociation, we know that multiplicity is found only within secondary and tertiary dissociation (OSDD-1, DID, P-DID), both levels of dissociation being caused solely by trauma, specifically repetitive early childhood trauma in OSDD-1 (secondary dissociation) and DID (tertiary dissociation). However, due to depersonalization, the other disorders encompassed by secondary and primary dissociation, as well as normal dissociation outside of disorders, may lead one to falsely believe they're multiple when they show no other symptoms of CDDs.
In the case of genuinely experiencing depersonalization that gives a false impression if multiplicity, it's a harmless mistake. It becomes ableist and harmful to the disabled community and yourself— even if unintentional— when you neglect to do research on the symptoms you think you could be experiencing (multiplicity in this case) and start spreading misinformation. Ableism isn't always intentional. That being said, these people should be treated with far more grace than intentional ableists (transplurals, willogenics, etc), because it was an honest mistake and they're clearly genuinely suffering from something, it just isn't multiplicity. They should be encouraged to seek help and do more research.
Obligatory mention of psychosis causing delusions of multiplicity. These people are suffering from delusions they can't control and aren't rational, therefore they cannot and should not be held accountable. You should gently encourage them to seek help, however.