Does HIPAA restrict a covered entity’s disclosure of PHI for treatment purposes to only those health care providers that are themselves covered by HIPAA?

No. A covered entity is permitted to disclose PHI for treatment purposes to any health care provider, including those that are not covered by HIPAA. In addition, HIPAA permits a covered health care provider to disclose PHI for the treatment of an individual to a third party, such as a social service agency, that is involved in the coordination or management of health care of that individual.