“women can't do basic math" being an antiquated notion is even weirder. As GP pointed out, companies used to employ lots of predominantly female computers. As a consequence, the first programmers and operators of digital computers were also predominantly women, even if the engineers building them were mostly men.

Women being bad at advanced math would make sense as an antiquated notion, but those in charge of hiring decisions until about 50 years ago evidently thought women were great at basic math.

The study you linked showing that women lag behind men in math to a degree proportional to some gender disparity metric is also interesting, but doesn't really tell us how we got here.