"Schizophrenia is defined as “a chronic mental illness in which a person loses touch with reality"." He didn't just want to fly, he thought he could actually fly. Therefore, he lost touch with reality. Although Diggs didn't seem to show other symptoms of paranoid schizophrenia, e.g. hallucinations, hearing voices etc. It's the most common form of the illness, meaning it could have just been a writers error.