IQ is just a measure of how fast a person can understand, abstract, and apply new information. It does nothing to examine how well someone understands a given subject, whether their conclusions are logical or accurate, or bothers to account for cultural and personal biases.
An IQ test is never a stand alone diagnostic tool in a comprehensive evaluation of a person's knowledge or ability. Its one of many metrics used to inform competent professionals of the capabilities of an individual in a controlled, non standard setting.