As doubly pointed out, the system is unlikely to be sane. In an insane system, you cannot predict that most employers will even know that intelligence is the best predictor of performance, let alone that they will effectively apply the best available method to select candidates by this criterion.

The fact is, from personal observation (which I admit is anecdotal evidence from a tiny, biased sample size), employers generally do not care to effectively figure this out. All employers I've encountered have had an attitude of wanting everything to "just work" (through the magic of being awesome, presumably) and land them the best employees because they will it to be so. If this would expand to the population in a proportional manner, it would mean that the vast majority of "employers" are either simply acting irrationally for this situation (AKA not only is the system insane, but nearly all its players are, too) or do not assign sufficient utility to obtaining better employees for it to be worth the perceived cost of finding them.

I believe this was the main point being made. It's not being argued that intelligence makes you a better actual performer, what is being argued is that employers do not effectively pick the most intelligent candidates, or worse, that they are not even remotely aware of what they should select for, and that they believe it is relatively worthless for them to attempt to find out more on this subject than they already know.