The main assumption fell on coal miners. Plumbers and auto mechanics (and now electricians?) and other blue-collar work came up as a generalization, though I'd generally accept the same bet -- that is, there probably won't be much mobility in out-of-work individuals in those fields to software (and IQ-gating is just one of several reasons why that would be since admittedly some BigCo tasks could be done by someone with an IQ between 80-90 but those people will almost never get in the usual way because to invert a tree on a whiteboard or whatever stupid proxy is in fashion probably requires an IQ of >100). But the main reason I assume most of those fields have a lower average IQ is because I assume their job tasks are less cognitively complex in a technical sense than those of a software engineer and I've read https://www.udel.edu/educ/gottfredson/reprints/1997whygmatte... Additionally an average programmer (maybe ignoring freelance web programmers?) has a CS degree and the average CS degree requires a certain level of rigorous math among other things that require above average intelligence to do well, whereas blue collar work typically requires nothing beyond a GED and sometimes various certifications or apprenticeships/training.

But why assume when we have data? Here's just one set. http://www.iqcomparisonsite.com/occupations.aspx If you find another that contradicts it please share.