No test measures intelligence. A test only measures you relative to the persons that wrote the test. – loosely quoting Asimov.
2007 is ancient history now. It is an interesting graph that one might correlate with a lack of meritocratic structure in society, but I’m on the low end cause I say this without looking up and reading the study. Pretty pictures evoke emotional blabbering bias and all that.
Not only that, on the other end of the graph it is known that poverty is a great stressor requiring constant mental bandwidth - that therefore can’t see used to “be smart”. So poorer people are not less smart, they allocated their mental resources to take care of their situation first and only later try to look smart in the test.