The field of psychology doesn’t always stick to scientific principles and is thus less reliable in diagnosing medical disorders. And there is a long standing rivalry between psychology and psychiatry. When working with a psychologist, it’s important to know their biases. For example, is this a psychologist who believes ADHD is not a real condition? Or that medication is not needed?
If you can establish that this person does adhere to scientific principles and is genuinely interested in diagnosing and properly treating ADHD (medication is the first-line treatment, so such a person would refer you to a doctor), then you should ponder the results seriously. Especially if there is a different diagnoses, ponder it. However, as mentioned above, if this person has a different cultural background than you, that could affect the accuracy of the assessment.
You can always get a second opinion, perhaps from a medical doctor, if you really can’t accept the current assessment (after serious pondering).