Another way to look at this would be to look at the correlation between scores of different standardized mental tests. ASVAB, IQ, and SAT scores are strongly correlated to one another, even though they use slightly different testing methodologies and focus in some ways on different things.

All of the evidence we've gathered over decades seems to indicate that there is a general factor for intelligence. But as he points out in his conclusion, this is /descriptive/, meaning we can arguably accurately determine someone's intelligence via testing and that single general factor accurately describes their intelligence in relation to others. It does not in any way imply we understand the causative factors that lead that score to go up or down in any meaningful way.

For reasons I don't fully understand, there's a lot of folks in the social sciences these day that seem to be waging war on the very concept that you can describe intelligence generally (and IQ tests specifically). The mere fact that the results of these types of tests on large population groups tend to follow a normal distribution is alone good evidence that they're indicative, as in most things diverse large population groups tend to fall on a normal distribution.