People who have good outcomes in life have a tendency to explain their choices in language around personal merit. Conversely, people who have poor outcomes have a tendency to explain their situations in language around personal demerit. The positive language often centers around "difficult educational attainment", "complex job function", "hard work", "ambition", and "intelligence", and the negative language often centers around "lack of ambition", "poor grades", "inability", and "lack of intelligence". As there is a statistically-significant difference between the IQs of people who have good outcomes, and those that have poor outcomes, it is very tempting to explain the differences in outcomes using IQ.
Now, it is impossible to measure anything objective about the mental faculties of a new-born child †. An 8-year-old can be assessed using a reading test, which measures what the child has learnt since birth. Because learning is ruthlessly cumulative, because neuroplasticity drops sharply with age, and because the access to opportunities later in life depend on assessments earlier in life, it should come as no surprise that children who do well in this test tend to have good outcomes later in life. Which children would do well in this reading test? A child who gets enough nutrition and medical care to develop fully; whose parents are able to afford the time and money to cook loving meals and take them to a good pediatrician; whose parents stimulate them by talking to them with a large vocabulary and correct grammar; whose parents nurture their curiosity with new and interesting topics; whose parents have enough free time outside work to spend with them; whose parents keep suitable toys and literature in the house; whose friends provide stimulating conversation; whose parents enroll them in a good kindergarten and school so that they have access to these friends; whose friends' parents are friends with their parents so they go on play-dates; whose parents live in good neighborhoods with easy access to all those things — the list could go on, and all be bucketed into a simple "socio-economic factors of family". Perhaps the best proof of this in action is the Great Gatsby Curve correlating income inequality with inter-generational mobility across different countries — the socio-economic factors play an outsize role in unequal countries, and indeed, inter-generational mobility is low in these countries. Correlating early-adult IQ, which is the end variable arising from two decades of consequences, with outcomes, is simply an obvious statement about how (mostly analytical) skill reflects in IQ. Explaining outcomes using IQ and bucketing professions by IQ to create an IQ-based caste system suffers from circular reasoning — perhaps it would be clearer to think of IQ not as "intelligence quotient", but rather as an abstract number that happens to be statistically correlated with outcomes in real life, whose utility is in decision-making around early intervention in children.
It follows from the discussion that the correct term to explain the difference in outcomes is privilege. The correct term for the job function of a high-skill worker is scarce, not hard or complex. They are highly rewarded by society simply due to demand-supply market forces, and justifications around their hard work are misplaced — low-skill workers work much longer hours and endure far more hardship. The inability of the vast majority of the people to perform a "complex" job function is the end result of being born unprivileged. If a barista or taxi driver had the same privilege as a lawyer, there would be no reason to doubt that they would have educated themselves at top universities, and been productive in a high-skill job function. In order to subvert the privilege explanation and explain differences in terms of personal agency and individual merit, it is necessary to invoke a dangerous eugenics argument, or a more benign luck argument — discounting privilege, the only difference between two 8-year-olds is genetics.