I've always had a firm grasp of vocabulary, grammar, and punctuation and don't have much education in the field. I think it is an indicator of one type of intelligence, but I also think the original question is flawed because there's really no such thing as a singular "intelligence". There's many, many types of intelligences, different "muscles" in the brain; some stronger than the other. I never liked the idea of an "IQ" for this very reason.