There main test has 60 datapoints... how many tries do you think they made exactly before deciding they didn't correlate as well? Yes, IQ tests are expensive, but if you buy one test, the marginal cost of giving to an extra student is basically 0. I suspect it wasn't tested at all.
A real IQ test takes at least 2 hours of 1-on-1 time with a psycholigist to administer, so in the event that someone actually wants to do real iq testing the marginal cost is quite high.
The ASVAB, GRE, LSAT, MCAT, and SAT all have correlations with psychologist administered IQ tests of around 0.9. WORDSUM, which is a ten word vocabulary test has a correlation of 0.71[1].
[1]Every time I use the WORDSUM variable from the GSS people will complain that a score on a 10-question vocabulary test is not a good measure of intelligence. The reality is that “good” is too imprecise a term. The correlation between adult IQ and WORDSUM = 0.71. The source for this number is a 1980 paper, The Enduring Effects of Education on Verbal Skills. I’ve reproduced the relevant table…