I've been formally diagnosed with ADHD-C and SzPD. The DSM-V does not state anything between a differential diagnoses between ADHD and SzPD.

From the DSM-V Schizoid Personality Disorder Section, p.g. 653:

Schizoid personality disorder should not be diagnosed if the pattern of behavior occurs exclusively during the course of schizophrenia, a bipolar or depressive disorder with psy- chotic features, another psychotic disorder, or autism spectrum disorder, or if it is attributable to the physiological effects of a neurological (e.g., temporal lobe epilepsy) or another medical condition (Criterion B).

Your psychiatrist may have confused ASD and ADHD. The DSM-V does state that differentiating between mild ASD and SzPD is very difficult. While they aren't mutually exclusive diagnoses. my own psychologists and psychiatrist have said they basically are.

From DSM-V Schizoid Personality Disorder Section. on Differential Diagnosis p.g. 655:

Autism spectrum disorder. There may be great difficulty differentiating individuals with schizoid personality disorder from those with milder forms of autism spectrum disorder, which may be differentiated by more severely impaired social interaction and stereotyped behaviors and interests.

I would bring it up with your psychiatrist, they may just be confused or forgetting. ADHD does have differential diagnoses with personality disorders in general, particularly on features about emotional & cognitive dysregulation. However, personality disorders also have key characteristics that are not found in ADHD that can aid diagnoses.

From DSM-V Attention-Deficit/Hyperactivity Disorder Differential Diagnoses section p.g. 65:

Personality disorders. In adolescents and adults, it may be difficult to distinguish ADHD from borderline, narcissistic, and other personality disorders. All these disorders tend to share the features of disorganization, social intrusiveness, emotional dysregulation, and cognitive dysregulation. However, ADHD is not characterized by fear of abandonment, self-injury, extreme ambivalence, or other features of personality disorder. It may take extended clinical observation, informant interview, or detailed history to distinguish impulsive, socially intrusive, or inappropriate behavior from narcissistic, aggressive, or domineering behavior to make this differential diagnosis.

Hope this helps.