Your question about how they measured “effectiveness” is right on.
My guess is that marginal benefits to IQ depend on the task, and the IQ range. For tasks of medium difficulty, the marginal benefits of IQ will probably increase as one goes from the low-IQ to average, flatten out and then decrease as one gets to a very high range. But higher IQ allows you to efficiently attempt more much more difficult (and arguably important) tasks.
Your question about how they measured “effectiveness” is right on.
My guess is that marginal benefits to IQ depend on the task, and the IQ range. For tasks of medium difficulty, the marginal benefits of IQ will probably increase as one goes from the low-IQ to average, flatten out and then decrease as one gets to a very high range. But higher IQ allows you to efficiently attempt more much more difficult (and arguably important) tasks.