First off, we don't have a good way to actually measure an individual's intelligence. IQ is actually meant to correlate with g which is a hidden factor we're trying to measure. IQ tests are good insofar as you look at the results of them from the perspective of a population. In these cases individual variation in how well it correlates smooths out. We design IQ tests and normalise IQ scores such that across time and over the course of many studies these tests appear to correlate with this hidden g factor. Moreover, anything below 70 and above 130 is difficult to measure accurately, IQ is benchmarked such that it has a mean of 100 and a standard deviation of 15. Below 70 and above 130 is outside of two standard deviations.
So, in summary, IQ is not a direct measure of intelligence. What you're doing here is pointing at some random guy who allegedly scored high on an IQ test and saying: "Look at how dumb that guy is. We must be really bad at testing."
But to say we don't know what intelligence is, is silly, since we are the ones defining that word. At least in this sense. And the definition we have come up with is grounded in pragmatism. The point of the whole field of research is to come up with and keep clarifying a useful definition.
Worth also noting that you can study for an IQ test which will produce an even less correlated score. The whole design and point of IQ tests is done with the idea of testing your ability to come up with solutions to puzzles on the spot.