The general concern with IQ when used for policymaking, or with standardized "intelligence" tests in general, is that they can fail to account for environmental or cultural factors across individuals or populations.

In both cases of covid and lead effects, the effect can be measured on the same population, and/or controlled for proxies of those factors (see "when controlling for age, gender, education level, income, racial-ethnic group and pre-existing medical disorders" in the article. So the general concern with the metric can be excluded.