If success is defined from an economic point of view in absolutes it makes sense. As the best predictor of your wealth is your parents wealth.
"Intelligence" is harder to add to the equation as it is more difficult to measure than parents wealth and there are known bias (https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect) that makes people think that they are smarter/more competent that they really are. So it is even possible that a superiority complex or insecurity are relevant even when the participants in the survey don't think so. (https://en.wikipedia.org/wiki/The_Triple_Package)
This results show the consequence of a diminishing socio-economic mobility. The same traits and skills in a high mobility society are going to have a big weight in "success" achievement. In an stagnated society where the system is rigged to make poor stay poor and rich stay rich "success" becomes an inherited trait making "socioeconomic background" the only and best predictor.