I'd think that it would be fairly easy to measure, honestly: do a longitudinal study of new hires at several top companies, and come back 5 years later and see who's still in the industry and what their level of promotion is if so. You know both their educational history, their demographics, and their standardized test scores (a close-enough proxy for g), and you can see how well they predict success.

There will be biases (in the scientific sense) in the data because of how the interview process selects for successful candidates, which is going to be correlated with these three and probably many other things. But I think the study would still be sound.

(Also, it occurs to me that I've been making about one assumption that is likely to be untrue: that people who do better at top software engineering companies are actually better software engineers. It is entirely possible that software engineering companies are systematically failing to hire and promote the best software engineers, because their interview and review processes are broken. In fact, that is what the pervasive-sexism-in-tech hypothesis implies: that individuals are so sexist that they would prefer to hire a man over a more qualified women, possibly because they're subconsciously failing to realize that the man is actually less qualified. If we want to test this hypothesis seriously, we need to separate "actually a good dev" and "recognized as a good dev". I could easily believe that being in the top percentile of g is correlated with hiring and promotion, because it's easy to measure, but less tightly correlated with actually being good at your job.)