The author should have defined the term "schizoid" since it colloquially refers to Schizophrenia rather than Schizoid Personality Disorder. The former involves hallucinations and is also a pejorative used to imply that someone is "crazy". The latter is an extreme form of introversion: https://en.wikipedia.org/wiki/Schizoid_personality_disorder
DSM-5 criteria:
- Neither desires nor enjoys close relationships, including being part of a family.
- Almost always chooses solitary activities.
- Has little, if any, interest in having sexual experiences with another person.
- Takes pleasure in few, if any, activities.
- Lacks close friends or confidants other than first-degree relatives.
- Appears indifferent to the praise or criticism of others.
- Shows emotional coldness, detachment, or flattened affectivity.
DSM-5 criteria: