The paper doesn't say IQ doesn't correlate to socioeconomic success, it just notes that IQ tests may be biased by socioeconomic status. But even within this study you could look at the lowest economic group, and the kids that have better genetic markers go to college at ~5x the rate of kids in the same socioeconomic group with poor markers. It also doesn't say much about access to college today for lower socioeconomic groups because it is entirely looking backwards at people born between 1905(!!) and 1964. It is not surprising that the lowest quintile is much less likely to have attended college. In 1940, the census bureau estimate is less the 5% of Americans had a bachelor's degree or higher, so it would be expected that the educated 5% was very biased towards the upper class.