What Gladwell is trying to say here is that though high IQ's are admirable, there comes a point when they have a very small impact on society. Meaning, 120 and 200 translate very similarily to the real world.
In the 20's, a psychology professor named Lewis Terman recorded the IQ's of 250,000 students across the US and found that about 1,470 of them had IQ's ranging from 140 to 200. He hypothesized that these few would be the next 'elite' of the United States. Well, he turned out to be wrong. Only a small selection did something with their lives, CEO's and politicians. But the rest remained very standard, and led ordinary lives.
Measuring intelligence branches beyond one's IQ. The Raven, a test that measures abstract reasoning
Another test called the Divergence test would be as simple as the one given in Outliers:
Write down as many diferecnt uses that you can think of for the following objects:
A brick
A blanket
This test would be based on creativity and improvising, something that an IQ test could never measure. This chapter states that a true genius is not measured on one intelligence, but more his overall potential as an outlier.