I agree with everything you say, but if >140 IQ accounts for the 99.5 percentile and true genius (in my estimation) is closer to 1 in 100000, then, even if every genius is within that IQ percentile, the predictive value is still essentially worthless.
Using those numbers, a random person has a 0.001% chance of being a genius. After they score >140 on an IQ test, this improves to a 0.2% chance. That's a big increase, but it isn't high enough to predict that an individual is a genius with any degree of certainty.
But, we can use the IQ test to predict who isn't a genius, or to make large scale predictions. For example, if a state has 100,000 students entering middle school per year, there's approximately one genius in that group. If you can afford an intensive math program for 7,500 students, the IQ test would let you predict that any potential geniuses get enrolled with high probability. That raises issues of fairness, discrimination, etc., but that's another topic.
