Actually, the DSM does have an exception for "culturally accepted" or "non-bizarre" delusions. It's pretty subjective and I imagine in practice the exceptions granted are mostly religious in nature, but there's definitely a level of acceptance past which the DSM wouldn't consider having a tulpa to be a disorder at all.

Furthermore, hallucinations are neither necessary or sufficient for a diagnosis of schizophrenia. Disorganized thought, "word salad", and flat affect are just as important, and a major disruption to the patient's life must also be demonstrated.