It's possible to be a very able, erudite person and not do well on an IQ test. Conversely, it's possible to score highly and be incompetent.

Intelligence is an ill-defined, nebulous quantity. IQ is a precisely defined quantity. Since intelligence is hard to measure and IQ is easy to measure, it's tempting to use one as a proxy for the other. But I, and the other posters above me, just don't find that very convincing.